ai 


Richard  M.  Holman 


-55*- 


BOTANY 


*Appleton\s     Scientific     ^Primers 
Edited  by J.^ynolds  Green,Sc.T>.,F.R.S. 

BOTANY 


BY 


J.  REYNOLDS  GREEN,  Sc.D.,  F.R.S. 

Downing  College,  Cambridge 


ILLUSTRATIONS 


New   Tork 

tAppleton  and  Company 


BIOLOGY 
LIBfcARY 


Apple  ton's  Scientific  Primers 

Edited  by 

J.    Reynolds    Green,   Sc.D. 

BIOLOGY.  By  Prof.  HARVEY 

GIBSON. 
CHEMISTRY.        By      Prof. 

W.  A.  TILDEN. 
BOTANY.     By  J.  REYNOLDS 

GREEN,  Sc.D:,  F.R.S. 


N\, 


PREFACE 

IN  writing  this  little  introduction  to  the  study  of  a  plant 
I  have  endeavoured  especially  to  present  it  to  the  reader 
as  a  living  organism.  Botany  is  now  regarded  as  a 
branch  of  biology,  and  is  not  satisfactorily  studied  by 
gathering  plants  and,  after  ascertaining  their  names  and 
the  natural  orders  to  which  they  belong,  drying  them 
and  putting  them  away  in  a  cabinet.  I  have  tried  to 
present  them  as  they  are  engaged  in  the  struggle  for 
existence,  and  to  call  my  readers'  attention  not  only  to 
their  form  and  structure  but  especially  to  what  they  do 
in  life,  and  why  and  how  they  do  it. 

I  hope  that  those  who  study  them  by  the  assistance 
of  this  little  primer  will  try  to  have  the  living  plant 
under  observation  whiler  they  read  it.  I  have  not 
written  any  detailed  scheme  of  laboratory  work,  but  I 
hope  my  readers  will  be  able  to  construct  such  a  scheme 
for  themselves  as  they  follow  the  directions  for  study 
given  in  the  text. 

I  should  like  to  suggest  that  students  should  read  the 
Chemistry  primer  first,  to  gain  some  acquaintance  with 
the  phenomena  underlying  the  processes  of  construction 
and  decomposition  going  on  in  the  plant.  It  would  be 
well  to  read  the  Biology  primer  also  before  beginning 
Botany. 

J.  REYNOLDS  GREEN. 

CAMBRIDGE,  1909. 


921904 


CONTENTS 


CHAP.  PAGE 

I.  INTRODUCTORY   ......  -7 

II.  THE  EARLY  DEVELOPMENT  OF  A  PLANT— THE  GERMINA- 
TION OF  A  DICOTYLEDONOUS  SEED 

III.  THE  FORMATION  OF  THE  ROOT  SYSTEM 

IV.  THE  STRUCTURE  OF  THE  ROOT    . 

V.  THE  CHARACTERISTIC  FEATURES  OF  THE  SHOOT    . 
VI.  THE  CONSTRUCTION  OF  THE  SHOOT  SYSTEM 
VII.  THE  STRUCTURE  OF  THE  SHOOT  . 
VIII.  THE  MONOCOTYLEDONOUS  PLANT 
IX.  THE  FOOD  OF  PLANTS 

X.  THE  RESPIRATION  OF  PLANTS      ..... 
XI.  THE  EVOLUTION  OF  THE  FORMS  OF  PLANTS — ALG.E 

XII.  THE  DEVELOPMENT  OF  THE  REPRODUCTIVE  PROCESSES  IN 
THE  ALG.E  ...  .... 

XIII.  THE  ORIGIN  OF  TERRESTRIAL  PLANTS  —  EVOLUTION  OF 

MOSSES  AND  FERNS      ...  .          . 

XIV.  REPRODUCTION    OF   FLOWERING    PLANTS  —  VEGETATIVE 

PROPAGATION        ....... 

XV.  THE  INFLORESCENCE  AND  THE  FLOWER 
XVI.   POLLINATION  AND  ITS  MECHANISMS — FERTILISATION 

XVII.  FORMATION    OF    THE    SEED    AND    ITS    MIGRATION — THE 
FRUIT  ........ 


VI 


BOTANY 

CHAPTER  I 

INTRODUCTORY 

OF  all  the  things  we  see  about  us  as  soon  as  we  escape 
from  the  life  and  surroundings  of  the  town,  none  is  more 
familiar  to  us  than  the  common  green  plant.  We 
tread  upon  grass  and  other  plants  which  clothe  the 
earth's  surface,  we  walk  under  trees,  around  bushes,  and 
by  the  sides  of  hedges,  or  we  wander  through  more 
cultivated  scenes,  enjoying  the  beauty  and  fragrance  of 
the  well-cared-for  garden.  In  all  this  wealth  of  vegeta- 
tion perhaps,  however,  one  fact  sometimes  escapes  our 
notice.  These  plants,  trees,  shrubs,  weeds,  or  what  not 
are  alive.  We  do  not  deny  this  when  we  hear  it  said, 
but  the  idea  is  hardly  a  prominent  one  in  the  view  we 
take  of  things  in  general.  It  is  based  probably  on  the 
fact  that  we  do  not  see  the  plants  move,  except  as  their 
slender  twigs  and  branches  or  their  numerous  leaves  are 
swayed  to  and  fro  by  the  wind,  for  to  our  own  some- 
what narrow  experience  life  is  so  closely  connected  with' 
restless  change  of  position  or  locomotion.  Yet  if  we 
wish  to  study  plants  to  learn  something  more  about 
them  than  a  casual  glance  can  tell  us,  we  must  bear  in 
mind  these  two  facts  on  which  their  whole  story  turns : 
first,  they  are  living  creatures ;  second,  they  spend  their 
lives  in  the  same  place  in  which  they  commenced  them. 
This  is  true  of  the  greater  number  of  plants  we  see 
around  us,  though  there  are  some  exceptions,  chiefly 

7 


8  BOTANY 

plants  which,  living  in  water,  are  passively  moved  about 
by  the  currents  of  the  stream. 

The  fact  that  a  plant  is  alive  and  conducts  itself  as  a 
living  organism  implies  certain  things.  It  must  receive 
suitable  and  sufficient  nourishment;  it  must  possess  a 
certain  power  of  adjusting  itself  to  its  surroundings, 
defending  itself  against  possible  dangers  and  over- 
coming definite  difficulties  which  these  surroundings 
occasion,  and  taking  advantage  of  such  benefits  as  are 
met  with  in  them.  It  must  possess,  to  at  any  rate  a 
limited  extent,  a  power  of  appreciating  its  relations  to 
such  surroundings,  of  realising  variations  in  certain  of 
them,  such  as  light,  moisture,  and  temperature,  that  it 
may  adapt  itself  accordingly. 

The  second  fact,  that  it  cannot  alter  its  position  by 
moving  freely  about,  makes  those  requirements  more 
essential.  It  also  demands  that  it  shall  be  possessed  of 
such  a  safe  attachment  to  its  situation  as  shall  secure 
it  an  appropriate  position  and  shall  enable  it  to  enjoy 
undisturbed  such  advantages  as  the  surroundings  offer. 
Further,  it  calls  for  a  certain  power  of  adjustment  of  its 
various  parts  to  the  air  above  it  and  the  earth  in  which 
it  is  fastened,  as  changes  in  both  of  them  are  frequent 
and  sometimes  violent.  As  the  only  sources  of  nourish- 
ment possible  to  it  are  the  air  and  the  soil,  together 
with  the  water  which  both  contain  in  constantly  vary- 
ing amount,  its  construction  must  be  such  that  the  same 
parts  which  secure  anchorage  or  support  shall  be  capable 
of  securing  supplies  of  the  various  materials  which  ulti- 
mately become  the  medium  of  nourishment. 

A  further  requirement  of  every  living  organism  is  the 
need  of  possessing  the  means  of  bearing  offspring  which 
shall  succeed  it  in  the  great  scene  of  nature.  To  a 
stationary  organism  this  introduces  difficulties  from 
which  the  readily  moving  animal  is  free,  but  these  diffi- 
culties have  been  overcome  by  adaptations  to  the  habit 


INTRODUCTORY 


of  life  which  are  among  the  most  complicated  and  the 
most  perfect  that  nature  shows  us. 

So  the  life  of  a  plant  shows  us  conflict  and  struggle 
waged  against  disadvantages  of  a  very  formidable 
nature;  a  power  of  appreciating  difficulties  and  of 
struggling  against  them ;  further,  it  exhibits  a  capacity 
of  seizing  upon  such  advantages  as  present  themselves, 
not  only  in  the  air  and  in  the  soil,  but  in  relative 
association  and  competition  with  each  other. 

We  are  familiar  with  the  fact  that  part  of  an  ordinary 
green  plant  is  embedded  in  the  soil.  Such  a  part  is 
commonly  known  to  us  as  its 
root,  and  we  distinguish  it  in 
several  ways  from  the  part 
which  rises  into  the  air  (Fig.  i). 
In  the  case  of  plants  which  live 
in  water  we  find  much  the  same 
division  of  the  plant  body. 
There  is  in  their  case  also  a  root 
part,  which  is  not  green  and 
which  is  buried  in  the  soil  or 
mud  at  the  bottom  of  the 
water  ;  there  is  a  part  which 
stretches  up  into  the  water,  in 
some  cases  extending  into  the 
air  above  the  surface.  We 
often  express  this  fact  by  say- 
ing that  the  plant  is  differenti- 
ated into  a  root  and  a  shoot. 

This  differentiation  is  a  funda-  FIG.  i.  Diagram  showing  the 
mental  one,  for  the  two  parts 
behave  very  differently.  They 
always  grow  in  opposite  directions,  and  as  these  direc- 
tions are  generally  upwards  and  downwards  they  are 
spoken  of  as  the  ascending  and  the  descending  axes  of 
the  plant. 


general     structure     of 
dicotyledonous  plant. 


io  BOTANY 

We  need  not  at  present  consider  very  fully  the  case 
of  the  water-plant,  and  will  therefore  examine  the 
relations  between  the  root  and  shoot  in  general  and 
the  surroundings  in  which  each  finds  itself. 

The  anchorage  of  the  plant  is  secured  by  the  penetra- 
tion of  the  soil  by  the  roots.  The  advantage  thus 
secured  is  not  obtained  without  difficulty  and  even 
danger.  To  become  fixed  in  the  soil  the  plant  must 
penetrate  it,  a  process  which  it  can  only  carry  out  by 
its  gradual  growth.  The  composition  of  the  soil  offers 
certain  difficulties  to  this  penetration:  it  may  be  too 
dense  or  too  powdery,  too  dry  or  too  wet ;  it  may  be 
slimy  like  clay,  or  very  hard  and  strong.  The  amount 
of  water  in  the  soil  and  the  degree  in  which  it  contains 
air  are  also  factors  which  must  be  taken  into  account 
in  considering  this  growth.  After  a  plant  has  once 
established  itself  and  secured  firm  anchorage,  it  still  has 
to  deal  with  varying  conditions  of  a  similar  nature,  for 
the  character  of  the  soil  is  very  liable  to  changes,  depend- 
ing on  conditions  of  temperature,  weather,  and  so  on. 

Besides  the  advantage  of  a  firm  anchorage,  the  root 
depends  upon  the  soil  for  the  supply  of  certain  materials 
which  ultimately  aid  in  some  way  in  its  nutritive  pro- 
cesses. Certain  minerals  are  necessary  to  every  green 
plant,  many  others  are  advantageous,  some  are  dele- 
terious. We  are  here  face  to  face  with  dangers  and 
advantages  which  need  adjustment  to  the  plant  as  it 
is  growing  in  the  soil.  Such  a  struggle  can  be  easily 
observed.  While  all  plants  need  compounds  of  nitrogen, 
some  will  only  flourish  on  soil  which  contains  as  well  a 
certain,  often  a  large,  proportion  of  chalk,  others  fail 
entirely  if  the  chalk  is  plentiful.  It  is  much  the  same 
with  other  constituents  of  the  soil. 

If  a  plant  is  growing  in  uncongenial  surroundings  it 
has  but  little  power  of  adjustment  to  them.  It  conse- 
quently dies  out  more  or  less  rapidly.  If  on  the  other 


INTRODUCTORY  n 

hand  its  environment  suits  its  constitution,  it  has  to 
adapt  its  structure  to  the  duty  of  absorbing  from  the 
soil  what  the  latter  will  afford.  So  the  two  duties  of 
anchorage  and  absorption  exist  together,  and  the 
differentiated  root  system  necessarily  discharges  both. 

If  we  turn  to  inquire  what  dangers  beset  the  part  of 
the  plant  we  have  called  the  shoot,  which  grows  up 
into  the  air  and  forms  a  head  that  is  frequently  of 
large  size,  we  find  them  taking  shape  in  the  various 
atmospheric  changes  incident  to  every  climate.  First 
of  these  we  may  place  wind  or  tempest.  As  the  shoot 
body  grows  it  must  offer  more  and  more  resistance  to 
air  currents,  a  resistance  which  may  easily  culminate  in 
a  violent  uprooting  of  the  plant.  This  involves  such  a 
subdivision  of  the  plant  body  as  will  allow  the  wind  to 
penetrate  through  it  without  serious  disturbance.  Here 
we  see  one  meaning  of  the  tapering  boughs  and  twigs, 
which  become  more  and  more  flexible  as  they  become 
increasingly  slender.  In  the  central  part  of  the  shoot 
system  they  are  rigid  and  can  resist  the  storm;  where 
by  their  dimensions  resistance  becomes  impracticable  we 
find  flexibility,  enabling  them  to  bow  to  the  wind  often 
so  completely  as  to  place  their  long  axes  parallel  to  the 
direction  in  which  it  is  blowing. 

Yet  another  reason  for  this  continued  subdivision  of 
the  plant  body  is  found  in  its  relation  to  the  absorp- 
tion from  the  soil  which  we  have  found  associated  with 
the  root.  The  latter  is  continually  absorbing  the  water 
of  the  soil ;  after  separating  from  such  water  the  mineral 
constituents  it  contains,  a  very  large  part  indeed  of  the 
water  is  evaporated,  and  so  passes  away  to  the  exterior 
again.  To  favour  such  evaporation  it  is  advantageous 
that  the  ratio  between  surface  and  bulk  shall  be  a  large 
one,  and  so  the  great  subdivision  of  the  subaerial  part 
of  the  plant  is  concerned  in  solving  the  problem  of  its 
nourishment. 


12  BOTANY 

Indirectly  the  composition  of  the  above-ground  part 
of  the  plant  has  a  direct  application  to  a  danger  to  which 
the  underground  region  is  exposed.  The  pressure  of 
the  wind  upon  an  unyielding  surface  in  the  air  would  be 
attended  by  great  danger  to  the  anchoring  root,  which 
might  be  violently  pulled  from  the  ground  by  the 
leverage  exerted  by  such  pressure.  The  great  subdivi- 
sion of  the  shoot  system  and  the  flexibility  of  its  ultimate 
twigs  minimises  this  danger,  but  even  as  it  is,  it  is  not 
unusual  after  a  tempest  to  find  even  sturdy  trees  up- 
rooted and  thrown  down. 

The  distribution  of  the  water  of  rain-storms  presents 
another  problem  which  must  be  solved  by  the  shoot 
system.  The  water  can  be  led  either  towards  or  away 
from  the  centre  of  the  plant.  Should  the  root  system 
be  one  which  spreads  considerably  and  extends  to  long 
distances  below  the  surface  of  the  soil,  it  is  of  great 
importance  that  the  rainfall  collected  on  the  central 
mass  of  shoots  shall  be  distributed  widely  so  as  to  reach 
as  far  as  the  extremities  of  the  roots,  watering  in  this 
way  a  large  area  of  ground.  If  the  root  system  consists 
of  a  strong  main  root  with  comparatively  few  branches, 
this  arrangement  would  largely  deprive  it  of  water. 
Hence  in  plants  with  roots  distributed  in  this  way  we 
find  arrangements  to  conduct  the  water  into  the  centre 
of  the  mass  of  shoots. 

In  some  rare  cases  the  duty  generally  discharged  by 
the  root  as  an  anchoring  organ  falls  upon  the  shoot, 
which  then  is  partly  developed  underground.  Such  a 
stem  bearing  in  its  turn  appendages  has  a  special  name 
—it  is  called  a  rhizome. 

If  we  pass  to  a  closer  study  of  the  much  divided  or 
branched  shoot  we  find  almost  invariably  that  its  ulti- 
mate twigs  put  forth  certain  regularly  arranged  flattened 
expansions.  In  cases  where  there  is  much  exposure  to 
currents  of  air,  these  flattened  portions  are  furnished 


INTRODUCTORY  13 

with  stalks  of  variable  length  which  are  extremely 
flexible  and  allow  the  flattened  organs  to  sway  freely 
backwards  and  forwards  as  the  wind  blows  upon  them. 
These  flattened  portions,  further,  are  usually  of  a  vivid 
green  colour;  they  are  then  known  as  leaves,  or,  pre- 
ferably, foliage  leaves. 

As  almost  all  plants  possess  leaves  we  may  inquire 
why  these  organs  should  so  uniformly  be  thin  and  flat. 

There  are  several  reasons  of  almost  equal  importance. 
The  leaf  or  other  winged  part  of  the  shoot  portion  is  in 
contact  or  relation  with  the  air  only.  Interchanges  of 
gases  between  the  air  and  the  leaf  are  continually  going 
on,  and  these  interchanges  are  effected  most  easily  and 
fully  with  a  large  extent  of  surface.  No  form  gives  so 
much  surface  in  proportion  to  its  bulk  as  a  thin  flat 
plate,  just  such  a  form  indeed  as  the  flattened  portion  or 
blade  of  the  leaf.  The  interchanges  include  the  absorp- 
tion of  particular  gases  from  the  air,  and  the  giving  out 
of  gases  and  water  vapour.  As  we  shall  see  later,  the 
internal  structure  of  the  leaf-blade  is  arranged  largely 
with  a  view  to  the  carrying  out  of  these  exchanges. 

A  second  reason  for  the  flattening  of  the  leaf  is 
concerned  with  the  manufacture  of  the  plant's  food. 
A  particular  gas  known  as  carbon  dioxide,  which  is  taken 
in  from  the  air,  is  ultimately  built  up  into  a  true  food 
material,  a  kind  of  sugar.  Though  the  formation  of 
sugar  in  the  plant  is  not  fully  understood,  it  is  known  to 
depend  upon  the  presence  of  the  green  colouring  matter 
and  its  being  properly  illuminated.  The  flattened  form 
helps  to  secure  the  arrangement  of  the  green  colouring 
matter  in  such  a  way  that  the  light,  either  of  direct 
sunshine  or  of  the  less  bright  diffused  daylight,  may 
reach  it  with  the  least  obstruction. 

Yet  a  third  reason  may  be  given.  The  leaves  are 
very  frequently  so  placed  that  they  extend  outwards 
from  the  plant  and  lie  nearly  parallel  to  the  surface  of 


14  BOTANY 

the  ground.  In  this  way  they  present  their  edges  to 
the  wind,  and  offer  as  little  obstacle  as  possible  to  its 
passage  through  the  tree,  so  making  as  small  as  possible 
the  risk  of  being  torn  off  when  the  force  of  the  wind  is 
strong.  As  the  wind  passes  between  them  they  are 
made  to  rise  and  fall,  but  they  offer  much  less  resistance 
to  its  force  than  they  would  if  they  were  not  flattened. 

The  arrangements  of  the  plant  and  its  parts  so  far  as 
we  have  studied  them  are  such  as  to  secure  its  firm 
attachment  to  the  soil,  its  stability  in  storms,  with  rela- 
tion both  to  wind  and  rain.  They  also  make  possible 
the  absorption  of  liquid,  containing  mineral  matters, 
from  the  soil ;  the  evaporation  of  the  excess  of  water  so 
absorbed ;  the  free  interchange  of  gases  between  it  and 
the  air;  the  needed  facilities  for  the  manufacture  of 
sugar  from  the  gases  absorbed  from  the  air  and  the 
water  from  the  soil.  They  are,  in  fact,  suitable  to 
support  and  nourish  a  stationary  living  organism  and 
to  furnish  defences  against  the  most  evident  dangers  to 
which  it  is  exposed. 

The  establishment  of  such  a  position  by  the  plant 
is  carried  out  by  means  of  growth  alone.  It  is  a  gradual 
process,  therefore,  and  must  be  accompanied  by  the 
nutritive  processes  which  enable  growth  to  take  place 
First  among  these  comes  the  supply  of  the  material  for 
the  increase  of  size  which  we  associate  with  growth. 
We  have  seen  that  the  plant  absorbs  from  the  soil  cer- 
tain mineral  compounds  dissolved  in  water,  and  from 
the  air  certain  of  its  constituent  gases.  The  most 
important  of  the  materials  which  the  earth  yields  are 
nitrates  of  potassium,  calcium,  and  other  metals,  phos- 
phates of  the  same,  traces  of  compounds  of  iron,  a  little 
silica  in  some  combination,  together  with  the  water  in 
which  they  are  dissolved ;  carbon  dioxide  is  supplied  by 
the  air.  When  the  absorption  of  these  substances  is 
possible,  and  when  light  is  sufficient  and  temperature 


INTRODUCTORY  15 

moderate,  the  healthy  plant  is  found  to  increase  in  size, 
and  gradually  to  show  all  the  phenomena  of  growth. 
Hence  these  various  compounds  have  been  regarded  as 
its  food.  This  is  not,  however,  a  correct  view,  for  they 
should  be  considered  only  as  raw  materials  from  which 
the  green  plant  can  make  the  food  it  needs.  This  is 
effected  by  the  agency  of  the  green  colouring  matter, 
the  so-called  chlorophyll,  but  only  when  it  receives  an 
appropriate  amount  of  light.  In  the  absence  of  chloro- 
phyll or  in  insufficient  light  the  supply  of  all  these 
various  compounds  does  not  afford  any  nourishment  to 
the  plant.  Plants  without  chlorophyll  are  not  far  to 
seek;  we  find  them  in  the  mushrooms,  in  the  moulds 
that  grow  so  readily  on  decaying  matter,  the  mildews  of 
corn  and  other  crops,  and  so  on.  These  cannot  develop 
at  all  when  supplied  only  with  the  inorganic  compounds 
mentioned. 

The  plant  then  in  order  to  grow  and  to  establish  itself 
has  to  be  provided  with  suitable  food.  If  it  has  chloro- 
phyll and  is  properly  illuminated  it  makes  this  food  for 
itself  from  the  inorganic  materials  the  soil  and  air  pro- 
vide. Plants  which  cannot  make  their  food  have  to 
obtain  it  from  living  or  dead  organic  matter.  Though 
this  is  difficult  it  is  not  impossible,  for  such  matter 
abounds  almost  everywhere — not  only  in  the  soil  but  in 
the  numerous  manufactured  products  which  we  meet 
with  all  around  us.  Living  organisms  also  are  often 
made  to  yield  food  to  these  non-green  plants.  The 
chlorophyll-containing  plants  are  continuously  making 
the  organic  substance  which  constitutes  their  food  as 
long  as  light  shines  upon  them.  We  find  them  growing 
at  its  expense  and  accumulating  large  quantities  of  such 
substances  as  sugar,  starch,  proteins,  and  fats  in  their 
own  bodies.  As  they  in  their  turn,  or  many  of  them, 
ultimately  become  the  food  of  animals,  we  may  see 
their  importance  in  the  role  of  nature.  The  fact  is  that 


16  BOTANY 

the  green  plant  is  the  only  organism  which  has  the 
power  of  forming  organic  substance  from  the  inorganic 
material  of  the  earth  and  air.  As  all  living  beings  are 
dependent  on  this  organic  substance  for  the  maintenance 
of  life,  we  see  how  the  continuation  of  life  itself  upon 
the  earth  depends  on  the  activity  of  the  green  plant. 

The  establishment  of  the  position  of  the  plant  and  its 
defence  when  so  established  may  be  seen,  therefore,  to  be 
subordinate  to  the  manufacture  of  organic  food. 

The  food  so  made  is  complex  in  character  and  will  be 
dealt  with  in  greater  detail  in  a  subsequent  chapter. 
It  comprises  chiefly  three  classes  of  substance:  carbo- 
hydrates, of  which  sugar  and  starch  are  representatives, 
fats,  and  proteins,  which  are  much  more  complex  in 
composition,  and  are  represented  by  the  white  of  egg 
and  by  the  chief  constituent  of  meat  and  fish.  The 
proteins  are  held  to  be  the  organic  material  which 
most  resembles  the  living  substance  itself. 

As  it  is  the  process  of  growth  at  the  expense  of  this 
newly  constructed  food,  or  of  a  small  supply  derived 
directly   from    its    parent,  by 
which  the  young  plant  makes 
its   way   into    its    appropriate 
position,  it  is  clear  that  this  is 
the  action  of  a  living  organism 
and  becomes  probable  that  the 
surroundings  of  the  plant  affect 
FIG.  2.   Geotropic  curvature  j1  in  other  ways  than  by  afford- 
in    root    and    shoot    of  ing  it  the  material  from  which 

(AfterrGibson.f Ufal  ^   to  make  its  food-     Careful  ob- 
servation  shows  that  this  is  the 

case.  The  root  of  the  plant  at  even  its  first  appear- 
ance grows  downwards  in  the  direction  of  the  soil. 
If  it  be  made  to  point  in  another  direction,  its  plan 
of  growth  slowly  changes  and  it  gradually  curves 
till  its  tip  is  pointing  downwards  again  (Fig.  2).  If 


INTRODUCTORY  17 

light  reaches  it,  it  bends  slowly  away  from  the  illu- 
minating ray;  if  anything  comes  into  contact  with  its 
tip,  growth  causes  it  to  curve  so  as  to  leave  the  obstacle 
on  one  side.  The  young  root  shows  in  these  ways 
certain  sensitivities,  reacting  to  the  incidents  of  its 
environment,  and  behaving  as  if  it  were  possessed  of 
rudimentary  perceptions  of  direction,  illumination,  and 
contact.  Other  features  of  the  environment  also  affect 
it,  particularly  moisture.  The  shoot  in  its  behaviour 
shows  similar  phenomena,  but  its  conduct  when  influ- 
enced, or,  as  it  is  generally  called,  stimulated,  by  gravity, 
light,  or  other  disturbing  causes,  is  as  a  rule  the  opposite 
of  that  of  the  root.  It  grows  upwards  against  gravity; 
it  curves  towards  and  not  away  from  light ;  its  behaviour 
with  regard  to  contact  is  not  always  uniform.  Both 
parts,  however,  show  what  we  claimed  at  the  outset  as 
one  of  its  primal  necessities,  the  power  of  adjusting 
itself  to  changes  in  its  surroundings. 

Consideration  of  the  fourth  requirement,  the  power  to 
reproduce  itself,  must  be  deferred  for  the  present. 

We  may  now  with  advantage  turn  to  the  composition 
of  the  plant  and  ask  what  is  the  distribution  in  it  of  the 
living  matter  to  which  this  behaviour  is  to  be  attributed. 

It  is  best  to  begin  the  study  of  this  point  by  examin- 
ing quite  a  young  plant,  or  preferably  the  seed  of  a 
plant,   as    the   structure   is    then   simple, 
while  it  becomes  very  complex  as  the  plant 
grows.     If  we  take  a  seed  (Fig.  3)  we  find 
it  contains  a  young  plant  or  embryo,  in 
which  bv  careful  dissection  we  can  make  J^****^  ^* 

J  ,  „,,        FIG.  3.  Section 

out  a  young  root  and  a  young  shoot.     The      Of    a    seed, 
shoot  consists  of  a  short  axis,  to    which      «>  embryo, 
are    attached   either   one    or    two   leaves 
known   as   cotyledons,    with   perhaps    traces    of   more 
leaves   above   them    (Fig.    4).     When   we   cut   such   a 
young  root  or  young  shoot,  we  find  that  it  is  made  up  of 

B 


i8 


BOTANY 


a  large  number  of  very  small  pieces  of  living  substance, 
or  protoplasm,  each  separated  from  its  neighbours  by  a 
thin  membrane  or  cell  wall  which 
surrounds  it  (Fig.  5).  Very  fine 
connecting  threads  of  protoplasm 
extend  through  the  cell  walls  and 
so  join  the  little  pieces  of  proto- 
plasm together,  but  these  are  so 
delicate  that  it  is  not  possible 

t0  See  the«  With°ut  Vefy  Skilful 

preparation. 

The  living  substance  thus  extends  throughout  the 
plant  in  complete  continuity,  though  it  is  apparently 
divided  into  a  number  of  separate  pieces  by  cell  walls  or 
membranes.  These  serve  at  the  outset  only  for  pur- 
poses of  support,  and  form  a  kind  of 
skeleton.  Each  little  piece  of  proto- 
plasm contains  a  small  highly  organ- 
ised portion  called  its  nucleus;  the 
whole  piece  is  called  a  protoplast ;  it 
is  approximately  cubical  in  shape  and 
has  a  diameter  of  about  i-30Ooth  of 
an  inch. 

As  the  little  protoplast  absorbs 
water  and  gets  larger,  entering  into 
active  life,  it  finds  itself  in  need  of 
constantly  renewed  supplies  of  water. 
Here  is  its  first  individual  difficulty, 
for  it  is  only  the  external  cells 
which  can  come  into  contact  with 
the  water  outside  the  plant.  To 
overcome  this  difficulty  the  proto- 
plast gradually  forms  a  central  cavity 
in  its  own  substance,  in  which  it 
holds  a  store  of  water.  This  cavity  is  known  as  a 
vacuole ;  it  is  of  the  greatest  importance  in  the 


FIG.  5.  Vegetable  cells. 
h,  celr.wall;  p,  proto- 
plasm; k,  k,  nucleus; 
s,  vacuole.  X7oo. 
(After  Sachs.) 


GERMINATION  OF  A  SEED  19 

maintenance  of  the  life  and  the  nutrition  of  the 
protoplast. 

As  the  plant  gets  older  and  larger  a  considerable 
amount  of  differentiation  of  its  internal  substance 
becomes  necessary.  This  we  shall  study  later.  Mean- 
time we  may  say  that  these  changes  are  accompanied 
by  the  death  of  some  of  the  protoplasts.  The  mem- 
branes or  skeleton  of  these  protoplasts  are  left  in  the 
interior  and  subserve  certain  important  purposes ;  but 
the  protoplasts  remain  in  full  vigour  towards  the 
exterior  and  particularly  towards  the  extremities  of 
both  shoots  and  roots,  where  new  formation  of  them  is 
continually  taking  place. 

The  living  substance  is  thus  situated  in  greatest 
amount  towards  the  outside  of  the  plant  and  at  its 
extremities,  where  its  contact  with  the  environment  can 
be  most  easily  maintained.  The  subordinate  mechanisms 
of  its  life,  which  are  concerned  with  its  mechanical  sup- 
port and  with  the  efficient  working  of  its  body  and  the 
co-ordination  of  its  various  forces,  are  hidden  away  more 
deeply  in  its  interior. 


CHAPTER  II 

THE   EARLY   DEVELOPMENT  OF   A   PLANT — THE    GERMINA- 
TION   OF    A   DICOTYLEDONOUS    SEED 

THERE  is  a  great  variety  in  degree  of  development 
among  the  plants  which  exist  upon  the  earth.  The 
most  highly  organised  of  these  are  the  so-called  flowering 
plants,  to  which  most  of  the  terrestrial  forms  belong. 

These  plants  have  a  certain  feature  in  common  which 
distinguishes  them  from  all  others.  They  form  seeds, 
which  become  separated  from  the  parent  and  after  a 
period  of  rest  develop  into  new  plants.  A  seed  is  essen- 
tially a  very  young  plant  in  a  dormant  or  resting 


20  BOTANY 

condition,  clothed  with  a  separable  protective  coat,  and 
supplied  with  a  certain  quantity  of  food  stored  in  it  or 
around  it  by  the  parent  from  which  it  came.  In  its 
quiescent  condition  this  young  plant  is  called  an  embryo. 
It  consists  of  a  young  root  and  a  young  shoot,  the  latter 
being  composed  of  a  stem  on  which  are  borne  a  certain 
number  of  leaves.  These  parts  are  known  as  the 
radicle  and  plumule  respectively,  the  first-formed  leaves 
being  called  cotyledons.  The  number  of  cotyledons 
varies ;  in  most  cases  there  are  two,  in  others  one,  while 
in  others  again  there  may  be  several.  The  number  of 
cotyledons  is  constant  throughout  large  groups  of  plants 
and  is  associated  with  differences  of  structure  of  the 
other  parts  of  the  plant.  The  first  two  groups  referred 
to  are  called  Dicotyledons  and  Monocotyledons.  In 
another  group  called  the  Gymnosperms  we  find  a 
variable  number,  sometimes  as  many  as  fifteen. 

The  young  embryo  is  fitted  to  bear  separation  from 
the  parent  and  transport  to  different  situations  by  the 
fact  that  its  life  is  in  a  dormant  state  and  that  it  is 
protected  by  the  skin  or  testa  of  the  seed.  Under 
appropriate  conditions  it  can  resume  active  life  and 
grow  into  an  adult  plant,  provision  having  been  made 
for  its  nutrition  during  the  early  stages  of  its  develop- 
ment and  until  it  acquires  the  power  of  making  its  own 
food.  This  necessary  food  is  prepared  by  the  parent 
plant  and  is  originally  deposited  as  a  relatively  bulky 
mass  around  the  embryo  in  its  early  development  in  a 
particular  cell  known  as  the  embryo  sac.  This  food  con- 
stitutes what  is  known  as  the  endosperm,  a  collection 
of  cells  which  fill  up  all  the  space  in  the  embryo  sac 
which  is  not  occupied  by  the  embryo. 

The  cells  of  the  endosperm  with  their  contents  are  all 
provided  for  the  nourishment  of  the  embryo.  In  some 
cases  the  embryo  feeds  upon  this  store  while  very 
immature  and  before  it  assumes  its  quiescent  state.  In 


GERMINATION  OF  A  SEED 


21 


others  its  quiescence  takes  place  very  early,  so  that  the 
endosperm  remains  unabsorbed  around  it  and  is  not 
used  till  the  resumption  of  active  life  and  growth  takes 
place.  The  difference  in  the  time  of  this  absorption 
influences  the  size  of  the  embryo,  which  is  naturally 
much  larger  when  it  has  absorbed  the  endosperm.  The 
food  so  absorbed  is  always  deposited  again  in  some  part 
of  the  young  embryo,  very  frequently  in  the  cotyledons 
which  become  large  and  fleshy.  Occasionally,  as  in  the 
Brazil  nut,  it  is  stored  in  the  axis  of  the  embryo.  . 

When  the  endosperm  persists  till  the  resumption  of 
life  by  the  embryo  —  the  process  known  as 
the  germination  of  the  seed  —  the  latter  is 
said  to  be  an  albuminous  seed  (Fig.  6).  If 
the  embryo  alone  is  present  inside  the  skin 
(Fig.  7)  it  is  called  exalbuminous. 

It  is  best  to  begin  the  study  of  these 
seedbearing  plants  with  the  largest  group, 
the  Dicotyledons. 

They  furnish  us  with  examples  of  both  classes  of 
seeds  which  are  easily  accessible  and  which  germinate 
readily.  We  may  take  first  the 
common  bean.  To  examine  the 
seed  it  is  well  to  soak  it  for 
several  hours  in  water,  which 
is  absorbed  by  the  skin,  so  that 
the  whole  seed  swells  and  its 

FIG.   7.      Embryo   of   pea  parts   can   be   easily  separated 
magnified,   r,  radicle;  nt,   from  one  another.      The  seed  is 

plumule;  c,  cotyledons.  .     .  .  ,  . 

somewhat  kidney  -  shaped,  and 

bears  on  the  concave  part  a  scar  at  the  point  at  which 
it  was  attached  to  the  fruit  from  which  it  came.  A 
little  way  from  one  end  of  this  scar  is  found  an 
aperture  through  the  skin,  known  as  the  micropyle, 
through  which  the  radicle  emerges  on  germination.  It 
can  be  localised  by  gently  squeezing  the  soaked  seed, 


a,  embryo. 


22  BOTANY 

when  a  drop  of  water  will  ooze  out  of  it.  On  removing 
the  testa  the  body  of  the  seed  is  found  to  consist  of 
a  very  bulky  embryo.  The  two  cotyledons  are  large 
masses  placed  face  to  face  and  easily  separated  from 
each  other.  On  gently  moving  them  apart  each  is  found 
to  be  attached  to  a  very  short  axis  which  lies  between 
them  and  is  almost  hidden  when  their  faces  are  in  con- 
tact. The  lower  end  of  the  axis  is  the  radicle  and  is 
bluntly  pointed ;  the  upper  end,  the  plumule,  which  curls 
inwards  between  the  cotyledons,  bears  two  minute  leaves. 

We  may  compare  with  the  bean  a  seed  of  about  the 
same  size,  that  of  the  castor  oil  plant.  It  must  be 
soaked  until  it  swells,  when  the  hard  coat  it  possesses 
will  crack.  On  removing  the  latter  a  fleshy  mass  will 
be  seen  which  cannot  be  separated  into  two  portions 
without  splitting  it.  If  it  is  divided  into  two  it  will  be 
iound  that  the  embryo  plant  consists  of  two  very  thin 
flat  cotyledons  lying  in  the  centre  face  to  face,  with  the 
very  short  axis  (plumule  and  radicle)  between  them. 
The  fleshy  part  of  the  seed  surrounds  the  whole  and 
adheres  firmly  to  the  backs  of  the  delicate  cotyledons. 
This  mass  is  the  endosperm,  which  has  not  been  absorbed 
by  the  embryo  during  its  early  growth. 

If  the  seed  is  soaked  in  alcohol  this  dissection  is  easier, 
as  the  parts  do  not  then  adhere  so  closely  together. 

After  a  period  of  variable  length  the  embryo  awakes 
from  its  quiescent  or  resting  state  and  develops  into  a 
seedling,  which  goes  on  to  become  an  adult  plant.  The 
quickening  into  this  renewed  activity,  which  is  techni- 
cally called  its  germination,  is  only  possible  when  the 
external  conditions  become  favourable.  The  process 
demands  moisture,  a  moderate  degree  of  warmth,  and 
the  presence  of  oxygen.  It  may  be  studied  easily  with 
a  little  care,  as  it  can  take  place  in  an  ordinary  room. 
The -absence  of  light  is  not  essential,  although  seeds  are 
usually  buried  in  the  soil  before  they  germinate. 


GERMINATION  OF  A  SEED  23 

Having  soaked  a  bean  for  several  hours  till  it  has 
become  swollen,  remove  it  from  the  water  and  keep  it 
on  damp  boiled  sawdust  or  in  some  moist  situation  in 
an  ordinary  room  for  some  days.  After  a  short  time  the 
young  radicle  will  be  found  to  protrude  from  the  micro- 
pyle  and  to  grow  downwards.  The  cotyledons  swell 
and  the  testa  cracks  and  begins  to  slip  off.  The  plumule, 
which  was  seen  to  be  curved  inwards,  elongates ;  the 
curvature  becomes  more  marked  and  forms  a  loop 
which  emerges  from  between  the  cotyledons ;  it  finally 
straightens  itself  and  thenceforward  grows  vertically 
upwards.  This  loop  is  formed  from  the  part  just 
above  the  cotyledons  which  is  known  as  the  epicotyl. 
The  cotyledons  remain  much  as  they  were,  but  as  the 
seedling  grows  their  contents  are  gradually  absorbed 
by  the  axis  and  they  shrivel  away.  In  their  normal 
development  when  the  seed  is  below  the  surface  of  the 
earth  the  cotyledons  remain  buried.  The  advantage  of 
the  looped  epicotyl  is  seen  as  it  presses  upward  through 
the  layer  of  soil  above  the  seed,  for  the  delicate  leaves 
•of  the  plumule  are  saved  from  the  injury  which  they 
would  suffer  if  they  had  to  force  their  way  through  the 
earth.  The  epicotyl  in  fact  opens  a  passage  for  them. 

Some  seeds  whose  structure  is  almost  identical  with 
that  of  the  bean  behave  a  little  differently  in  germina- 
tion. The  part  of  the  axis  which  elongates  and  brings 
the  plumule  through  the  soil  is  a  region  a  little  below  the 
cotyledons,  and  it  is  consequently  called  the  hypocotyl. 
The  lengthening  of  this  part  causes  the  cotyledons  also 
to  be  carried  up  into  the  air,  and  after  a  short  time  they 
turn  green,  and  take  on  the  work  of  the  foliage  leaves 
which  are  developed  as  the  plumule  grows. 

When  the  castor  oil  seed  germinates  the  early  stages 
are  much  the  same  as  in  those  of  the  bean.  The  seed 
swells  and  the  radicle  grows  through  the  micropyle,  and 
very_soon  the  young  root  branches  freely.  The  endo- 


24  BOTANY 

sperm  swells  and  the  flat  cotyledons  which  remain  in 
contact  with  it  begin  to  absorb  the  contents  of  its 
cells.  The  face  of  the  endosperm  becomes  very  slimy 
or  mucilaginous  and  it  continues  to  swell  for  some 
days,  ultimately  cracking  and  being  loosely  attached 
to  the  absorbing  cotyledons.  The  hypocotyl  grows  up 
in  the  form  of  a  loop  and  drags  the  cotyledons  out  of 
the  soil  with  the  endosperm  clinging  to  them.  They 
very  quickly  change  colour,  becoming  yellow  and  ulti- 
mately green,  and  as  the  last  traces  of  the  endosperm 
are  used  up  they  grow  out  laterally  and  take  on  the 
appearance  and  the  function  of  foliage  leaves. 


CHAPTER  III 

THE   FORMATION   OF   THE   ROOT   SYSTEM 

THE  seeds  just  described  are  very  useful  for  observing 
also  the  growth  and  development  of  the  seedling.  Even 
better  material  for  this  purpose  is  supplied  by  the  seeds 
of  the  common  cress.  If  several  of  these  seeds  are 
soaked  in  water  and  then  scattered  over  the  inside  of  a 
damp  flower  pot  they  will  germinate  very  freely  if  the 
pot  is  kept  moist  and  moderately  warm,  putting  out 
their  roots  in  a  few  hours.  As  they  will  have  been  sown 
quite  indiscriminately,  their  positions  will  be  irregular 
and  the  young  rootlets  will  emerge  at  first  in  very 
different  directions.  If  they  are  allowed  to  remain  un- 
disturbed as  they  elongate  they  invariably  manage  to- 
direct  their  apices  downwards,  effecting  sometimes 
curious  curvatures  to  do  so.  This  strange  uniformity 
of  behaviour  suggests  that  the  young  seedling  has  a  kind 
of  appreciation  of  its  position  or  the  direction  of  its 
growth.  We  can  test  this  suggestion  by  taking  several 
of  them  from  the  positions  they  have  assumed  and 
placing  them  so  that  their  roots  are  at  different  angles 


THE  FORMATION  OF  THE  ROOT  SYSTEM     25 

with  the  vertical.  So  long  as  they  are  intact,  they 
gradually  modify  their  growth  so  as  to  make  their  apices 
again  point  vertically  downwards  (Fig.  8). 

If  we  study  the  behaviour  of  the  roots  under  various 
conditions  we  soon  find  that  they  manifest  other  forms 
of  sensitiveness,  all  of  which  are  brought  to  bear  upon 
the  problem  of  establishing  themselves  in  the  soil. 
When  a  root  enters  the  latter  and  passes  between  the 
particles  which  compose  it,  it  must  sooner  or  later 
come  into  contact  with  some  of  them,  and  not  improb- 
ably such  contact  will  hinder  the  advance  of  the  root 
in  a  straight  or  nearly  straight 
line.  The  growth  of  the  root  is 
achieved  by  its  advancing  in  a 
kind  of  corkscrew  fashion,  the 
tip  describing  a  spiral  rather 
than  a  straight  line.  This  no 
doubt  tends  to  push  aside  slight 

obstacles  which  may  meet   the    FIG.  8.  Geotropic  curvature  in 

advancing  tip.      If  we  experi-      root  and  shoot  of  mustard. 


ment   upon   a   seedling    bean,  size'>       (After 


which  we  have  seen  can  be  culti- 
vated in  moist  air,  we  can  imitate  the  conditions  met 
with  in  the  soil  by  attaching  some  small  piece  of  a  hard 
substance  to  one  side  of  the  root  tip,  using  a  little  gum 
as  the  attaching  medium.  By  this  treatment  we  can 
ensure  that  the  contact  shall  be  prolonged,  and  hence 
the  struggle  between  the  root  and  the  obstacle  will  be 
carried  to  such  a  point  as  to  exhibit  very  striking  effects. 
After  a  short  time  the  growing  region  of  the  root,  which 
is  some  little  distance  behind  the  apex,  will  be  observed 
to  curve  in  such  a  way  as  to  turn  the  tip  from  the  object 
touching  it.  As  the  pressure  is  not  removed  under  the 
conditions  of  the  experiment,  this  curvature  will  become 
very  pronounced  and  after  a  day  or  two  the  root  will  be 
curled  into  a  loop.  In  the  soil  so  pronounced  a  curva- 


26  BOTANY 

ture  is  not  met  with,  as  a  slight  change  in  the  direction 
of  growth  causes  the  root  to  grow  past  the  obstructing 
body,  and  then  the  downward  direction  is  resumed. 

We  can  thus  show  that  the  young  root  has  not 
only  an  appreciation  of  direction,  but  it  can  in  some 
way  recognise  when  it  is  in  contact  with  some  solid 
obstacle  and  that  it  can  modify  its 
growth  with  a  view  to  getting  past  such 
a  body  and  penetrating  further  into 
the  soil. 

The  root  further  appreciates  the  in- 
cidence of  a  lateral  light.  If  the  seed- 
ling is  cultivated  in  a  glass  vessel  and 
so  placed  that  light  reaches  it  only  on 
one  side  it  very  quickly  modifies  its 
growth  so  that  the  apex  becomes  turned 
away  from  the  light.  In  the  soil  this 
behaviour  brings  it  closer  to  the  par- 
ticles of  the  soil,  especially  a  little  way 
behind  the  tip.  These  three  rudimen- 
tary  senses  or  sensitivities  are  supple- 
™nted  by  a  fourth.  It  shows  an 
evident  appreciation  of  the  presence 
of  moisture,  and  grows  towards  the  dampest  parts  of  the 
medium  in  which  it  is  placed. 

If  we  revert  for  a  moment  to  the  young  cress  seedling 
we  find  that  when  it  has  attained  the  length  of  about 
half  an  inch  a  number  of  long  delicate  outgrowths  of 
its  surface  may  be  seen  arranged  in  a  broad  band  all 
round  the  root  at  a  little  distance  behind  the  apex  (Fig. 
9).  So  long  as  the  root  grows  this  band  of  outgrowths, 
which  are  known  as  root  hairs,  is  maintained.  New  ones 
are  formed  on  the  side  of  the  apex  while  the  older  ones 
die  and  disappear  on  the  hinder  margin  of  the  band. 
As  the  root  advances  in  the  soil  these  hairs  become  so 
closely  attached  to  its  particles  that  they  cannot  be 


THE  FORMATION  OF  THE  ROOT  SYSTEM     27 

separated  mechanically.  While  they  thus  aid  materi- 
ally in  attaching  the  root  to  the  soil,  they  carry  on  the 
absorption  of  the  water  of  the  soil  with  the  mineral 
compounds  dissolved  therein. 

It  is  customary  to  consider  the  influences  we  have 
spoken  of,  gravitation,  contact,  light,  and  moisture,  as 
stimuli,  and  to  speak  of  the  behaviour  of  the  root  as 
•response  to  stimulation.  The  power  of  receiving  stimu- 
lation indicates  the  possession  of  special  sensitiveness, 
and  its  response  is  to  a  large  extent  under  the  control 
of  the  living  root.  The  movements  or  alterations  of 
growth  are  purposeful,  and  lead  us  to  look  upon  the 
latter  as  a  living  sensitive  organism  engaged  in  the  task 
of  making  the  best  of  its  surroundings  and  varying  its 
behaviour  as  the  surroundings  change. 

Seeing  the  very  purposeful  behaviour  of  the  root  we 
may  pause  to  ask  what  is  the  most  potent  factor  in  the 
growth,  or  what  is  the  determining  influence  which 
causes  it  to  penetrate  the  ground.  The  fact  that 
stability  of  position  is  secured  strikes  us  at  once,  but  it 
is  doubtful  if  this  is  the  first  consideration. 

We  may  dismiss  the  responses  to  the  stimulation  of 
light  and  contact.  They  are  accessory  to  the  effort  of 
the  plant  to  come  into  close  relatio'n  with  the  soil,  but 
they  by  themselves  do  not  minister  to  any  of  its  needs. 
The  behaviour  of  the  root  suggests  that  it  is  seeking 
something  which  the  long  experience  of  the  race  has 
shown  to  be  advantageous  and  which  has  now  become 
hereditary  in  the  plant.  The  object  of  this  search  is 
the  water  which  the  soil  contains,  which  is  present  as 
delicate  films  surrounding  the  particles  of  which  it  is 
composed.  Inherited  experience  has  shown  to  the 
vegetable  organism  that  the  soil  is  the  source  of  water, 
and  its  instinctive  efforts  are  directed  to  the  securing  of 
a  position  leading  to  an  adequate  supply. 

The  stimulus  of  gravity,  therefore,  or  the  perception 


28 


BOTANY 


of  direction,  indicates  to  the  root  the  whereabouts  of  the 
water  which  it  needs.  The  perception  of  water  aids  that 
of  direction  and  under  normal  conditions  the  two  co- 
operate. If,  however,  there  be  no  water  in  the  soil,  the 
inherited  instincts  of  the  plant  lead  it  to  penetrate  even 
the  driest  sand. 

If  the  plant  is  in  such  a  position  that  the  two  stimu- 
lations do  not  co-operate,  but  are  antagonistic  to  each 
other,  the  chief  instinct  of  the  plant  becomes  evident, 
and  it  can  be  shown  that  its  great  object  is  the  coming 
into  relationship  with  water  rather 
than  with  soil. 

If  some  seedlings  are  allowed  to 
grow  on  a  sieve  which  is  covered 
by  a  layer  of  moss  they  will  at  the 
outset  put  out  their  roots  through 
the  holes  of  the  sieve  and  grow 
downwards  in  a  normal  way,  seek- 
ing as  their  inherited  instincts  tell 
them  the  soil  which  should  nor- 
mally be  situated  below  them.  If 
the  sieve  is  suspended  over  a  pan  of  water,  so  that  moist 
air  is  below  the  roots,  they  keep  on  growing  downwards  as 
if  growing  into  earth.  But  if  the  conditions  be  changed 
after  the  roots  have  attained  a  length  of,  say,  half  an 
inch,  the  air  below  them  being  made  very  dry  by  artificial 
means,  while  the  moss  in  the  sieve  is  kept  well  wetted,  the 
roots  soon  curve  upwards  and  growing  in  opposition  to 
gravity  turn  towards  the  water  (Fig.  10).  They  appear 
to  recognise  that  their  original  instinct  is  deceiving 
them  and  that  the  true  habitat  for  them  is  for  some 
reason  above  and  not  below  them.  If  after  they  have 
established  this  new  direction  of  growth  the  conditions 
be  again  changed  and  moisture  be  restored  to  the  air 
below  them  while  the  moss  is  allowed  to  dry,  another 
reversal  of  the  direction  of  growth  takes  place  and 


FIG.   10.    Hvdrotropism 
(After  Gibson.) 


THE  FORMATION  OF  THE  ROOT  SYSTEM     29 

again  the  position  of  the  water  determines  this  direction. 

The  behaviour  of  the  root  thus  shows  it  to  possess 
certain  tendencies  which  are  based  upon  inheritance  of 
the  accumulated  experience  of  the  race  to  which  it 
belongs,  but  which  are  controlled  by  certain  sensitivities 
which  are  its  own  personal  possession.  These  sensi- 
tivities are  no  doubt  hereditary  also. 

The  power  of  appreciating  the  influence  of  these 
various  stimulating  influences  has  been  found  to  be 
confined  to  a  very  small  region  of  the  root,  extending 
about  one-tenth  of  an  inch  from  the  apex.  This  region, 
which  may  be  called  the  root  tip,  may  consequently 
be  regarded  as  a  rudimentary  sense  organ.  There 
is,  however,  nothing  in  its  structure  to  mark  it  off 
from  the  region  further  back.  The  part  receiving 
the  stimulus  is  not  the  part  which  becomes  curved 
in  the  act  of  responding.  The  latter  is  the  region  of 
active  growth,  where  the  cells  are  undergoing  elonga- 
tion. The  cells  at  the  tip  only  retain  their  sensitiveness 
for  a  short  time.  When  new  cells  are  formed  in  front  of 
them  in  the  process  of  elongation  these  are  found  to  be 
sensitive,  and  the  original  ones,  passing  into  the  region 
of  active  growth,  lose  the  power  of  appreciating  stimula- 
tion. There  is  thus  no  permanent  sense-organ  in  the 
root.  The  protoplasm  is  sensitive  at  some  particular 
stage  of  its  development,  and,  having  passed  that  stage, 
loses  its  power  of  appreciating  these  stimulating  changes. 

The  way  in  which  the  stimulus  received  at  the  tip 
causes  a  modification  of  the  growth  of  the  cells  some 
little  distance  farther  back  is  not  at  present  understood. 
Something  in  the  nature  of  a  nervous  impulse  is  thought 
to  be  transmitted  from  the  one  region  to  the  other, 
passing  along  the  delicate  threads  of  protoplasm  which 
extend  through  the  separating  walls  of  the  cells  and  put 
all  the  cells  in  communication  with  one  another. 

We  noticed  in  studying  certain  seedlings,  especially 


30  BOTANY 

those  of  the  castor  oil  plant,  that  the  root  does  not 
remain  single  but  very  speedily  begins  to  give  off 
branches.  By  this  process  of  branching  a  very  large 
root  system  is  made  possible.  The  main  root  of  Di- 
cotyledons usually  persists  and  remains  longer  and 
stronger  than  its  branches.  Such  a  main  root  is  called 
a  tap  root.  The  branches  in  turn  branch  and  we  get 
roots  of  the  second,  third,  and  higher  orders.  If  we 
trace  the  formation  of  these  roots,  as  we  can  do  by 
cultivating  a  seedling  in  water,  or  a  dilute  solution  of 
the  necessary  mineral  compounds,  we  find  that  they 
arise  in  constant  succession  as  the  main  root  grows, 
the  youngest  thus  being  always  nearest  the  growing 
point  of  the  main  root.  Each  branch  root  has  the  same 
appearance  as  the  one  from  which  it  springs,  and 
similarly  bears  near  its  apex  a  band  or  zone  of  root 
hairs.  The  branches  orginate  in  the  interior  of  the  old 
root  and  bore  their  way  outwards.  They  arise  in 
definite  positions,  in  relations  to  certain  internal 
structures  which  will  be  discussed  a  little  later. 

The  branches  are  sensitive  to  the  same  stimuli  as 
the  main  root,  but  they  respond  rather  differently  to 
the  action  of  gravity.  Instead  of  growing  vertically 
downwards,  the  first  branches  stand  out  nearly  at  right 
angles  to  the  main  root  and  persist  in  growing  in  this 
direction.  The  branches  which  in  their  turn  they  bear 
do  not  grow  in  such  definite  positions,  but  extend 
symmetrically  round  the  one  from  which  they  spring. 
If  by  accident  the  main  root  is  killed,  its  place  is  taken 
by  one  of  its  strongest  branches,  which  alters  its  response 
to  gravity  and  grows  vertically  downwards. 

By  this  course  of  development  the  root  system  of  a 
plant  comes  to  occupy  considerable  space  in  the  earth 
and  to  fill  the  interstices  of  the  latter  very  completely. 
Two  advantages  are  thus  secured:  a  very  firm  grip  of 
the  soil  is  secured  by  the  attachment  of  the  root  hairs 


THE  STRUCTURE  OF  THE  ROOT  31 

of  the  numerous  rootlets,  spread  through  so  much  of 
the  earth,  aided  very  conspicuously  by  the  large  net- 
work which  the  branches  form;  and  a  very  large  area 
of  water-covered  particles  is  tapped  by  the  absorbing 
root  hairs  the  rootlets  bear. 

As  the  system  gets  older,  not  only  is  it  continually 
enlarged  by  the  increased  branching,  but  the  individual 
roots  and  branches  increase  in  girth  and  press  more  and 
more  firmly  into  the  soil.  They  penetrate  very  deeply 
and  extend  laterally  very  widely,  so  that  with  the  in- 
creasing size  of  the  above-ground  portion  of  the  plant  a 
firmer  and  firmer  anchorage  is  afforded,  securing  the 
needed  stability. 


CHAPTER  IV 

THE    STRUCTURE    OF   THE    ROOT 

THE  internal  structure  of  the  root  can  be  properly 
understood  only  when  it  is  studied  from  the  point  of 
view  of  the  work  which  the  root  has  to  do.  At  its  first 
emergence  from  the  seed  its  substance  is  composed  of  a 
large  number  of  the  vegetable  cells  which  we  have 
described,  each  a  little  mass  of  protoplasm  separated 
from  its  neighbours  by  delicate  cell  walls.  They  are  in 
close  contact  with  each  other  at  all  points  and  have  no 
cavities  in  them.  The  chief  difference  in  the  mass  is 
that  the  external  cells  at  the  apex  form  a  kind  of  cap 
over  the  tip  of  the  radicle,  so  that  its  actual  apex  is  not 
exposed.  This  cap  protects  the  true  apex  from  damage 
as  it  penetrates  into  the  soil.  When  the  radicle  has 
begun  to  elongate  changes  in  the  cells  are  set  up.  If  a 
longitudinal  section  of  it  (Fig.  n)  is  examined  these 
changes  will  be  seen  to  separate  the  young  root  into 
roughly  three  areas.  The  cap  can  be  seen  in  front,  a 
short  region  behind  it  shows  the  cells  small  and  actively 


BOTANY 


dividing,  so  increasing  their  number,  and  a  longer  part 
still  further  back  is  marked  by  the  enlargement  of  the 

cells  in  all  directions,  but 
most  notably  longitudinally, 
while  their  vacuoles  are  being 
formed.  These  regions  are 
known  as  the  root  cap,  the 
region  of  cell  division,  and 
the  region  of  cell  growth. 
Little  more  can  be  distin- 
guished at  this  stage. 

A  little  later,  when  the  ex- 
ternal band  of  root  hairs 
appears,  preparation  for  the 
discharge  of  particular  duties 
by  the  different  parts  begins 
to  be  indicated,  while  the 
requirements  of  the  life  of 
the  organ  involve  further 
adaptations.  The  first  of 
these  is  the  admission  of  air 
to  the  interior  to  supply  the 
oxygen  all  living  substance 
needs  to  breathe.  The  com- 
mencement of  the  formation 
of  an  aerating  mechanism 
can  be  traced  all  through  the 
young  embryo,  even  at  this 
age ;  as  seen  in  an  older  root 
it  consists  of  the  splitting  of 
the  cells  apart  from  one 
another  at  some  point  of 
each,  frequently  at  the  angles 
their  walls  make  with  each 
other  (Fig.  12).  These  little  splittings  make  a  number 
of  spaces  between  particular  cells,  and  as  growth  goes  on 


FIG.  ii.   Longitudinal  section  of 
young  root.     Xao. 


THE  STRUCTURE  OF  THE  ROOT  33 

these  separate  spaces  become  united,  so  that  intercellular 
passages  run  among  the  cells  of  every  region,  being  of 
different  dimensions  in  different  areas.  As  we  shall  see 
later  these  passages  become  open  to  the  exterior  in  the 
upper  portion  of  the  plant  and  so  enable  air  to  enter  and 
circulate  in  the  interior  of  the  tissues. 


enpeph  x    p     x     px 

PIG.  12.  Section  of  central  part  of  root.  In  the  outer  region  the  cells 
are  separated  in  places  by  the  intercellular  spaces.  en,  endo- 
dermis;  pe,  pericycle;  ph,  phloem  strand;  p,  pith;  x,  xylem 
strand;  px,  protoxylem.  Xioo.  (After  Kny.) 

A  longitudinal  section  of  the  root  taken  at  this  age 
will  show  that  beside  the  longitudinal  areas  or  regions 
already  remarked  the  internal  tissue  is  beginning  to  be 
differentiated  in  another  direction.  The  section  of  the 
root  is  almost  conical,  but  the  apex  of  the  cone  can  be 
divided  into  three  layers,  each  of  which  is  continued 
backwards  alone:  the  axis.  At  the  apex  each  layer  can 

c 


34  BOTANY 

be  recognised  in  the  zone  of  cell  division.  The  cells  of 
these  layers  can  divide  and  they  are  called  in  conse- 
quence meristematic  layers.  The  outermost,  which  is 
known  as  the  dermatogen,  forms  the  root  cap,  and  ex- 
tending backwards  gives  rise  also  to  the  outermost 
layer  of  the  root  from  which  the  root  hairs  grow.  The 
central  one  forms  a  more  or  less  well-marked  cylinder 
or  core,  which  is  known  in  the  meristematic  region  as 
the  plerome,  while  the  intermediate  one  is  called  the 
periblem,  and  forms  the  part  of  the  root  that  lies 
between  the  central  cylinder  and  the  external  layer.  As 
we  trace  these  further  backwards  we  find  that  the 
central  cylinder  becomes  very  clearly  marked  off  from 
the  rest  by  a  peculiar  layer  called  the  endodermis. 

The  root  hairs  are  long  slender  outgrowths  of  the  cells 
of  the  outer  layer,  which  when  past  the  meristematic 
region  is  known  as  the  pili/erous  layer,  or  epiblema. 
Each  hair  has  a  thin  wall  of  cellulose,  which  is  brought 
into  close  contact  with  particles  of  soil  as  it  grows  in 
among  them.  On  coming  into  contact  with  these 
particles  the  outer  layers  of  its  walls  become  changed 
into  a  kind  of  mucilage,  which  makes  the  hair  adhere 
very  closely  to  the  soil.  The  film  of  water  which  sur- 
rounds the  particles  is  then  absorbed  by  the  root  hair. 
As  there  are  enormous  numbers  of  these  hairs  on  the 
young  root,  there  is  soon  a  great  increase  in  the  water 
which  the  root  contains.  This  water  passes  on  from 
the  hairs  into  the  second  region  of  the  root,  now  called 
cortex  instead  of  periblem,  and  gradually  makes  its  cells 
extremely  swollen  or  turgid  thereby. 

The  special  mechanism  for  carrying  this  water 
from  the  root  to  the  upper  parts  of  the  plant  is  by  this 
time  beginning  to  appear.  It  lies  in  the  central  region, 
now  partly  shut  off  from  the  rest  by  the  endodermis. 
Here  the  growth  of  the  cells  is  such  as  to  cause  them 
to  become  elongated.  Certain  special  areas  of  these 


THE  STRUCTURE  OF  THE  ROOT 


35 


elongated  cells  form  a  definite  number  of  columns  of 
cells  which  can  be  traced  separately  upwards.  They 
are  fitted  especially  to  transport  water  by  changes  in 
the  constitution  of  their  cell  walls,  which  become 
gradually  changed  from  cellulose  to  lignin,  the  latter 
enabling  water  to  pass  through  it  in  all  directions  with 
great  ease.  At  the  same  time  the  horizontal  walls  of 

st      st  h  TV     C' 


FIG.  13.  Longitudinal  section  through  a  vascular  bundle  of  a  stem. 
5,  s',  p,  p.  different  types  of  wood  vessel;  w,  wood  fibres;  st,  sieve 
tubes;  ph,  bast  fibres;  p',  pith;  c,  cambium. 

these  cells  in  great  part  disappear,  so  that  the  columns 
of  cells  become  changed  into  hollow  tubes,  or  vessels, 
while  their  side  walls  are  irregularly  thickened  by  the 
deposit  of  more  cell-wall  substance  upon  them  in 
particular  areas.  On  account  of  the  presence  of  these 
vessels,  the  collections  are  known  as  vascular  strands  or 
vascular  bundles  (Fig.  13).  In  the  root  they  are  com- 
posed, entirely  of  lignified  cells  and  are  therefore  called 
wood  or  xylem  bundles,  to  distinguish  them  from  other 
vascular  strands  lying  near  them.  The  number  of  these 


36  BOTANY 

strands  varies  in' different  roots ;  it  is  very  common  to 
find  four,  but  two  is  not  an  infrequent  number.  They 
may  extend  completely  to  the  centre  and  all  unite  there 
to  form  a  solid  cylinder.  If  the  number  is  large  they 
.generally  fuse  together  before  extending  so  far,  leaving 
a  small-celled  column  as  a  core.  This  is  known  as  a 
-pith.  In  form  the  bundles  are  wedge-shaped,  the  apex 
of  the  wedge  pointing  outwards. 

If  we  trace  these  conducting  strands  towards  the  tip 
•of  the  root  they  can  be  distinguished  among  the  soft 
•cells  of  the  plerome  by  their  narrow  diameters  and  their 
tendency  to  elongation.  The  area  of  each  embryonic 
strand  can  be  seen  distinctly  in  a  transverse  section, 
their  small  size  and  a  certain  density  of  their  protoplasm 
marking  them  off  from  their  neighbours.  The  gradual 
change  from  these  cells  to  the  mature  forms  can  be 
traced;  the  alteration  of  the  wall  and  its  thickening 
appear  first  along  the  outer  edge  of  the  wedge,  known 
consequently  as  the  protoxylem,  and  extending  thence 
towards  the  centre  of  the  root. 

If  these  vascular  bundles  are  traced  along  the  root  in 
the  direction  opposite  to  the  tip  they  are  seen  to  be 
continuous  with  similar  structures  in  the  stem.  In  this 
way  a  path  is  made  throughout  the  plant  for  the  trans- 
port of  the  water  after  its  absorption. 

These  strands  are  chiefly  concerned  with  the  func- 
tion of  the  root.  Others  which  also  are  traceable 
throughout  the  plant  can  be  seen  to  lie  one  between 
each  pair  of  them  in  the  central  cylinder.  These  are 
chiefly  concerned  in  the  nutrition  of  the  root.  They 
are  equally  well  defined  and  lie  side  by  side  with  the 
wood  strands,  separated  from  them  by  a  few  packing 
cells.  They  differ  in  texture,  their  walls  remaining 
cellulose.  They  are  known  as  bast  or  phloem  ;  and  are 
made  up  of  vessels  known  as  sieve  tubes  from .  their 
terminal  walls  being  somewhat  thickened  and  perforated 


THE  STRUCTURE  OF  THE  ROOT 


37 


by  a  number  of  holes,  so  that  their  protoplasm  is  con- 
tinuous (Fig.  13).  With  the  sieve  tubes  are  a  certain 
number  of  slightly  elongated  cells  of  the  ordinary  type. 
The  bast  and  wood  strands  are  thus  seen  to  occupy, 
with  a  little  supporting  tissue,  almost  the  whole  central 
cylinder  of  the  root  (Fig.  14).  There  is  always  an  outer 


FIG.  14.  Section  of  central  part  of  root,     b,  bast  strands;  w,  wood 
bundles.     Xioo.     (After  Kny.) 

continuous  sheath  over  the  whole,  one  cell  thick  as  a 
rule,  which  is  called  the  pericycle.  Outside  the  peri- 
cycle  comes  the  endodermis. 

The  endodermis  forms  a  sheath,  one  cell  thick,  round 
the  central  cylinder.  Its  walls  in  some  cases  become 
uniformly  thickened  and  lignified.  In  others  the 
outer  and  inner  walls  remain  thin,  while  the  side  walls 
become  changed  in  a  different  way.  The  cellulose  is 
replaced  by  another  material  which  resists  rthe  passage 


38  BOTANY 

of  water  through  it,  so  that  the  water  of  the  cortex  can 
pass  directly  to  the  wood  strands,  but  cannot  pass  from 
one  endodermal  cell  to  another,  being  prevented  by 
bands  of  a  cuticularised  substance  that  pass  round  the 
radial  walls  (Fig.  15).  By  their  interlocking  together 
they  make  the  endodermis  separate  the  intercellular 
passages  of  the  cortex  from  those  of  the  cylinder,  so  that 
air  cannot  penetrate  directly  to  the  latter. 

As  the  root  grows  older  and  larger  and  the  upper  part 
or  shoot  system  of  the  plant  develops  to  a  corresponding 
extent,  this  primary  structure  becomes  insufficient  for 

its  requirements.  They  call 
for  a  greater  amount  of 
conducting  tissue  as  the 
branches  and  leaves  of  the 
shoot  multiply,  for  all  the 
latter  need  a  supply  of 
water.  The  stability  of 
the  whole  structure  needs 

pericycie.  °  strengthening,  in  view  of  the 

greater  size  being  acquired 

above  ground.  There  is,  as  we  have  seen,  a  great  growth 
in  thickness  of  the  root  and  the  development  of  a 
system  of  branches,  each  behaving  like  the  parent  root. 
In  the  stage  we  have  examined  the  young  root  shows 
no  provision  for  this  increase  of  thickness.  It  can  take 
place  only  by  the  formation  of  new  cells,  and  such  forma- 
tion is  not  going  on  except  at  the  apical  meristem.  A 
new  departure  has  accordingly  to  be  made  (Fig.  16). 
It  begins  by  a  curved  band  of  cells  of  the  supporting 
tissue  lying  in  front  of  each  strand  of  bast  becoming 
meristematic,  beginning  to  divide  by  walls  which  are 
parallel  in  direction  with  the  circumference  of  the  root. 
These  tangential  divisions  cause  the  formation  of  several 
rows  of  cells,  one  of  which,  the  nearest  to  the  bast, 
retains  the  power  of  division  and  is  called  cambium. 


THE  STRUCTURE  OF  THE  ROOT 


39 


The  newly  formed  cells  become  converted  into  wood, 
so  that  a  strand  of  wood,  called  secondary  wood,  is 
formed  inside  each  bast  bundle.  The  cambium  layer 


FIG.  1 6.  Thickening  of  root ;   px,  primary  wood ;   .SAT,  secondary 
wood;   c,  cambium;   en,  endodermis.      X8o.     (After  Kny.) 

extends  laterally  round  the  bast  bundle,  so  that  it  tends 
to  pass  up  towards  the  outer  edge  of  the  wood  bundle  on 
each  side.  By  the  time  a  little  mass  of  wood  has  thus 
been  formed  between  each  bast  strand  and  the  centre 
of  the  root,  the  cells  of  the  pericycle  outside  the 
wood  bundles  divide  by  similar  tangential  walls,  so 


40  BOTANY 

that  the  pericycle  at  these  points  becomes  several  cells 
thick.  The  innermost  of  these  cells,  lying  in  contact 
with  the  protoxylem,  become  cambium,  and  soon  extend 
to  unite  the  two  bands  of  cambium  approaching  them 
from  the  two  bast  strands  between  which  the  bundle  of 
wood  is  lying,  so  that  a  complete  ring  of  cambium  is 
formed.  At  first  it  is  necessarily  sinuous  or  wavy,  but 
as  more  and  more  wood  is  formed  inside  the  bast  masses 
it  is  pushed  further  and  further  outwards  there,  till  the 
waviness  of  the  ring  disappears.  This  cambium  ring 
then  continues  to  add  more  and  more  wood  in  the  same 
way  to  the  secondary  wood  already  formed.  Behind 
the  protoxylem  groups,  which  form  the  outer  edge  of 
the  primary  wood  bundles,  no  secondary  wood  is  formed, 
but  only  rows  of  thin-walled  cells;  consequently  the 
secondary  wood  is  divided  into  separate  masses  by 
these  rows  of  cells,  which  are  known  as  medullary  rays. 
They  are  formed  with  a  view  to  the  transport  of  food 
substances  from  the  bast  into  the  interior  of  the  wood. 

The  cambium  produces  a  little  secondary  bast  out- 
side the  ring  in  the  same  way  as  it  forms  wood  inside  it, 
but  the  quantity  of  bast  is  much  less  than  that  of  wood. 
This  is  natural,  as  the  bast  has  only  to  provide  a  path 
of  transit  for  the  actual  food  of  the  root  cells,  while  the 
wood  has  to  furnish  a  continually  increasing  amount  of 
water-transporting  tissue. 

This  woody  formation  in  the  centre  of  the  root  is  dis- 
posed very  advantageously  for  maintaining  its  stability. 
A  structure  with  a  hard  central  core  is  the  most  suitable 
to  resist  such  a  vertical  pull  as  would  cause  uprooting. 
This  vertical  pull  is  continually  being  made  by  the 
movement  of  the  storm-tossed  upper  region  of  such  a 
structure  as  a  tree. 

The  young  root  as  it  increases  in  thickness  in  the  soil 
encounters  two  dangers,  one  internal,  the  other  external. 
The  process  of  thickening  compresses  very  severely 


THE  STRUCTURE  OF  THE  ROOT  41 

its  more  external  layers  and  in  time  ruptures  them- 
The  pressure  of  wet  soil  against  its  epiblema  is  not 
unlikely  to  set  up  decay.  The  cortical  tissues  and  the 
epiblema  are  therefore  inadequate  to  protect  the  gradu- 
ally thickening  central  cylinder.  But  these  difficulties 
become  obviated  as  the  growth  proceeds.  By  the  time 
the  central  cylinder  has  become  only  slightly  thickened 
the  zone  of  the  root  hairs  has  been  removed  to  some 
distance  in  advance,  by  the  continuous  elongation  of  the 
root.  The  cortex  of  the  thickened  part  is  consequently 
not  supplied  with  water  as  before,  and  ceases  to  play  its 
original  part  in  transporting  the  water  upwards.  The 
hairs  having  disappeared  from  that  region  too,  the 
epiblema  has  not  its  first  importance  there.  The  pres- 
sure of  the  gradually  increasing  girth  stimulates  the 
cells  of  the  pericycle  and  they  again  show  the  power  of 
increasing  by  tangential  divisions.  The  pericycle  be- 
comes uniformly  several  cells  thick,  one  layer  of  which 
remains  meristematic.  It  cuts  off  repeatedly  bands  or 
shells  of  cells  which  remain  very  regular  in  shape,, 
appearing  in  transverse  sections  like  rows  of  bricks. 
The  outermost  ones  lose  their  contents  and  their  walls 
are  transformed  into  suberin,  a  substance  closely  re- 
sembling the  cuticularised  material  of  the  endodermis. 
This  band  of  cells  forms  what  is  known  as  a  cork  layer. 
It  extends  completely  round  the  root  and  forms  a 
strongly  protecting  sheath.  It  is  perforated  here  and 
there  by  little  rounded  masses  of  cells  loosely  arranged1 
so  that  air  can  pass  between  them.  These  are  known  as 
lenticels ;  they  serve  to  admit  air  to  the  interior  of  the 
root.  It  is  quite  impervious  to  water  except  at  these 
spots,  and  hence  preserves  the  root  from  loss  of  water  by 
outward  leakage.  The  cells  of  the  cortex  and  epiblema 
may  now  rot  away  without  causing  any  damage  to  the 
root.  The  latter  acquires,  in  fact,  a  fresh  exterior  of 
a  more  resistant  and  permanent  character  than  the 


BOTANY 


PIG.  17.  Transverse  section  of  root  to  show 
a  rootlet  at  two  stages  of  development. 
rh,  root  hairs ;  ec,  cortex ;  d,  cells  in  pro- 
cess of  absorption;  en,  endodermis;  pe, 
pericycle;  co,  conjunctive  tissue;  ph, 
bast ;  g,  cambium ;  x,  wood ;  c,  derma- 
togen  of  rootlet ;  p,  its  periblem ;  pi,  its 
plerome.  (After  Scott.) 


original  one.  This 
corky  formation  con- 
tinues as  long  as  the 
root  lives  and  adapts 
itself  to  its  increasing 
girth.  Its  outer  part 
is  composed  of  dead 
cells,  and  together 
with  the  remains  of 
the  layers  originally 
outside  it,  constitutes 
the  bark  of  the  root. 
The  cortex  and  epi- 
blema  continue  for  a 
very  short  time,  so 
that  in  an  old  root  the 
bark  consists  of  peri- 
cycle  tissue  and  layers 
of  cork. 

We  must  again  re- 
turn to  the  young  root 
to  trace  the  manner 
of  formation  of  its 
branches.  The  latter 
originate  when  it  is 
quite  young,  as  we 
have  seen  already. 
They  arise  in  the  peri- 
cycle,  in  very  many 
cases  opposite  to  the 
protoxylem  of  each 
wood  bundle,  gener- 
ally before  the  strands 
are  lignified  through- 
out. There  are  con- 
sequently usually  as 


THE  SHOOT  43 

many  rows  of  lateral  roots  as  there  are  wood  strands. 
A  little  group  of  the  cells  become  marked  out  by  be- 
coming meristematic,  and  dividing  chiefly  by  tangential 
walls,  so  that  soon  a  little  mass  seems  to  be  growing 
outwards.  It  can  shortly  afterwards  be  seen  to  have  a 
central  plerome  covered  by  a  periblem  and  dermatogen, 
which  behave  just  like  those  of  the  parent  root.  The 
cells  of  the  cortex  which  lie  in  front  of  the  new  root 
branch  are  gradually  digested  and  eaten  by  the  latter  as 
it  grows  outwards  and  finally  penetrates  to  the  exterior 
(Fig.  17). 

The  cells  of  the  root  cap  are  continually  being  worn 
away  by  contact  with  the  soil.  The  cap  is  added  to  all 
the  while  by  the  dermatogen  behind  it. 


CHAPTER  V 

THE  CHARACTERISTIC  FEATURES  OF  THE  SHOOT 

THE  work  which  falls  upon  the  shoot  portion  of  the 
plant  is  very  different  from  that  discharged  by  the  roots, 
being  very  largely  the  construction  of  the  organic  sub- 
stance which  serves  as  food,  not  only  for  the  plant  itself 
but  for  the  world  in  general.  To  understand  this  con- 
struction we  must  consider  the  absorption  of  carbon 
dioxide,  the  utilisation  of  certain  amounts  of  the  water 
and  mineral  constituents  furnished  by  the  roots,  and 
the  evaporation  of  the  surplus  water.  The  work  in- 
volves certain  minor  or  subordinate  duties  connected 
with  the  distribution  of  the  food  after  its  formation. 

The  important  questions  of  the  breathing  of  the  plant 
and  the  maintenance  of  a  suitable  temperature  in  its 
different  parts  must  also  engage  our  attention. 

The  form  and  composition  of  the  shoot  need  careful 
study  from  these  points  of  view,  but  these  are  not  all. 
The  relation  of  its  structure,  internal  as  well  as  external, 


44  BOTANY 

to  its  stationary  position,  and  the  difficulties  and 
dangers  which  the  latter  presents,  must  be  considered. 
The  adaptations  which  it  shows  and  the  changes  of 
climate  which  it  meets  are  of  great  importance. 
Finally,  we  have  the  relation  of  the  shoot  system  to  the 
processes  of  reproduction. 

When  the  young  shoot  has  emerged  from  the  seed  and 
made  its  way  into  the  air  in  the  ways  already  described, 
the  bent  or  hooked  form  gradually  changes  till  an  up- 
right position  is  attained.  We  have  already  examined 
the  behaviour  of  the  young  root,  noting  its  perception  of 
direction  and  its  modification  of  its  growth  if  necessary, 
till  it  can  make  its  way  vertically  downwards.  The 
same  appreciation  of  direction  is  exhibited  by  the  young 
shoot  and  its  behaviour  is  very  similar,  with  the  im- 
portant difference,  however,  that  it  seeks  the  light  and 
air  and  hence  grows  vertically  upwards.  We  cannot 
explain  this  difference  except  by  recognising  the  pur- 
poseful character  of  its  response  to  the  influence  of 
gravity.  There  is  no  difference  in  the  growing  cells,  so- 
far  as  we  can  see,  for  they  have  all  practically  the  same 
structure  whether  they  are  in  root  or  shoot.  We  see  in 
this  behaviour  really  a  living  organism  trying  in  a. 
limited  way  to  make  the  best  of  the  circumstances  in 
which  it  finds  itself.  As  we  continue  to  study  it  we 
shall  be  able  to  ascertain  that  it  possesses  the  same 
sensitivities  and  powers  of  response  to  changes  in  its 
surroundings  that  we  have  found  exhibited  by  the  root. 

The  growth  of  the  shoot,  however,  is  a  much  more 
complicated  process  than  that  of  the  root,  in  conse- 
quence of  its  more  manifold  duties,  which  have  called 
for  a  more  complicated  structure. 

The  young  plumule  when  it  has  emerged  from  the 
seed  coats  consists  of  a  very  delicate  axis,  at  the  apex 
of  which  a  number  of  minute  outgrowths  are  to  be  seen. 
These  are  folded  in  various  ways,  the  outermost  covering 


THE  SHOOT  45 

those  internal  to  them.  Their  number  is  not  uniform, 
nor  is  their  method  of  folding,  nor  their  arrangement, 
but  they  all  arch  over  the  apex  of  the  shoot.  The 
latter  does  not  bear  any  protective  cap,  such  as  is  seen 
over  the  root.  It  is  a  delicate  conical  tip,  which  bears 
its  outgrowths  in  regular  succession,  the  latter  being 
continually  developed  by  the  apex  as  it  elongates,  so 
that  the  youngest  are  always  nearest  to  the  tip. 

These  outgrowths  are  borne  upon  the  axis  at  definite 
points,  which  show  a  remarkable  difference  of  behaviour 
from  the  spaces  between  them,  in  that  they  do  not 
elongate  during  the  processes  of  growth.  All  the  growth 
in  length  is  carried  out  by  these  spaces.  The  points  at 
which  the  outgrowths  are  borne  are  called  nodes,  the 
spaces  between  them  internodes. 

The  behaviour  of  these  parts  can  be  studied  advan- 
tageously on  a  shoot  a  little  older  than  the  plumule. 

It  is  well  to  select  a  tree  of  some  few  years'  growth  and 
to  examine  some  of  the  ultimate  endings  of  its  branches. 
If  from  such  a  tree  in  the  early  spring  we  take  a  twig 
we  shall  be  able  to  observe  that  during  the  previous 
summer  its  internodes  elongated,  causing  the  out- 
growths to  be  separated  from  each  other  by  some  little 
distance.  The  year's  growth  may  have  caused  the  shoot 
to  become  perhaps  three  or  four  inches  long.  If  we 
examine  the  nodes  closely  we  shall  find  that  between 
the  original  outgrowths  and  the  axis  certain  small  knob- 
like  bodies  occur,  almost  hidden  between  the  others. 
These  several  parts  can  always  be  observed  with  greater 
or  less  facility  on  all  shoots.  The  axis  is  called  the 
stem,  the  first-formed  outgrowths  are  the  leaves,  and  the 
little  knob-like  bodies  between  the  two  are  known  as 
buds.  The  angle  between  the  stem  and  its  leaf  in  which 
the  bud  arises  is  the  axil  of  the  leaf.  The  apex  of  the 
stem  will  be  seen  in  the  spring  to  exhibit  also  the  form 
of  a  bud,  rather  larger  than  the  lateral  ones  in  the  axils 


46  BOTANY 

of  the  leaves.  The  plumule  is  really  the  first  bud  of  the 
seedling,  and  it  shows  fundamentally  the  same  structure 
as  the  others  appearing  later  on  the  stem. 

As  the  seasons  of  the  year  in  our  climate  render 
growth  intermittent,  confined  to  little  more  than  half 
the  time,  and  as  the  growing  shoots  are  exposed  to  very 
unfavourable  conditions  during  the  remainder,  it  is  easy 
to  understand  that  special  precautions  are  called  for, 


A  B 


FIG.  1 8.  Buds  of  lilac.  A,  shows  the  external  appearance; 
B,  a  slightly  magnified  section ;  C,  the  bud-scales  are  reflexed 
and  the  leafy  shoot  has  begun  to  elongate.  (After  Marshall 
Ward.) 

that  they  may  develop.  If  we  cut  a  longitudinal 
section  through  one  of  these  buds  in  the  spring  before 
growth  is  resumed  we  shall  find  evidence  of  such  (Fig. 
1 8  B).  The  delicate  growing  cone  in  the  centre  will  be 
found  to  be  surrounded  by  a  varying  number  of  leaves, 
each  of  which  arches  over  it  and  is  in  turn  arched  over 
by  the  next  one  external  to  it.  The  most  internal  ones 
are  extremely  delicate  and  almost  unformed,  while  the 
cone  itself  if  magnified  will  be  seen  in  many  cases  to  bear 
upon  its  surface  small  swellings  which  indicate  that 
other  leaves  are  in  course  of  formation  there.  Over 


THE  SHOOT  47 

these  delicate  leaves  are  others  more  sturdy,  while  the 
exterior  ones  are  frequently  quite  dry  and  hard  and  in 
many  cases  covered  over  by  a  sticky  substance.  Some 
of  those  in  the  interior  are  in  many  cases  covered  with 
thick  coatings  of  hairs,  forming  a  downy  pad  of  material 
calculated  by  its  non-conductivity  to  keep  out  the  cold. 

If  the  bud  is  small,  it  will  be  found  to  contain  only  a 
few  leaves,  perhaps  only  two  or  three;  even  in  this 
case,  however,  the  general  arrangements  are  the  same. 

If  we  compare  the  apices  of  stem  and  root,  we  see 
how  the  surroundings  in  each  case  have  influenced  the 
structure.  The  root  apex  is  specially  protected  against 
damage  from  contact  with  hard  or  rough  materials  while 
penetrating  through  the  soil ;  the  stem  is  exposed  to  no 
such  danger,  but  shows  a  careful  protection  from  frost 
and  wet,  and  undue  evaporation. 

The  young  leaves  are  thus  merely  flattened  boat- 
shaped  expansions  curling  over  the  apex  of  the  stem. 
Later,  when  their  protective  powers  are  no  longer 
called  for,  their  adult  forms  are  assumed. 

The  leaves  bring  about  their  curving  over  the  apex  of 
the  stem  in  the  bud  by  an  irregularity  of  growth.  When 
the  little  swelling  first  appears  on  the  growing  cone  it 
is  itself  rounded  or  conical;  it  soon  becomes  laterally 
flattened  and  for  a  time,  so  long  as  it  is  in  the  bud,  its 
under  surface  grows  faster  than  its  upper  one,  so  that  it 
is  made  to  curve  forwards.  When  it  escapes  from  the 
bud  later  it  reverses  this  distribution  of  growth  and 
grows  more  rapidly  on  its  upper  face,  so  becoming  flat. 

Buds  always  terminate  the  ends  of  normal  growing 
shoots ;  indeed  the  bud-form  is  always  assumed  by  the 
apex  of  the  shoot  as  soon  as  its  growth  is  suspended  by 
unfavourable  conditions.  The  buds  which  appear  in 
the  axils  of  the  leaves  lower  down  on  the  stem  are  the 
commencements  of  the  secondary  shoots  or  branches, 
which  will  elongate  in  due  course. 


48  BOTANY 

In  many  cases  the  bud  is  the  foreshadowing  of  the 
growth  of  the  stem  or  branch  of  the  next  year.  It  has 
"been  formed  by  the  shoot  as  its  last  effort  for  the  year, 
and  its  development  during  the  succeeding  year  will 
only  involve  the  elongation  of  the  internodes,  the 
assumption  of  the  adult  forms  of  the  leaves,  and  the 
preparation  of  the  buds  for  the  year  following.  In 
other  cases  it  is  not  so  simple.  During  the  growing 
period  more  leaves  will  be  produced  than  the  bud  in  its 
resting  state  exhibits,  and  growth  will  be  prolonged 
accordingly.  But  even  in  these  cases  as  soon  as  growth 
in  length  stops,  the  development  of  another  terminal 
bud  with  its  potentialities  can  be  noticed. 

The  growth  of  the  shoot  thus  shows  considerable 
•differences  from  that  of  the  root.  In  the  case  of  the 
latter  it  is  not  at  all  easy  to  say  what  are  the  limits  of 
the  year's  elongation,  while  in  that  of  the  shoot  they 
may  be  fairly  accurately  determined. 

When  the  next  growing  season  sets  in,  the  bud  begins 
to  swell  owing  to  the  upward  pressure  of  the  elongating 
axis.  The  outer  leaves  are  loosened  and  pressed  apart, 
•so  that  the  bud  bursts  open  at  the  apex.  When  the 
external  leaves  are  hard  scales  they  are  generally  cast 
off  entirety,  and  the  internal  leaves  emerge.  The 
^elongation  of  the  several  internodes  rapidly  follows  and 
the  shoot  takes  on  its  proper  form. 

As  this  change  proceeds  certain  other  facts  can  be 
determined.  The  external  scales  have  no  buds  in  their 
axils,  nor  do  all  the  leaves  develop  into  foliage  leaves. 
The  external  ones,  and  often  some  just  internal  to  them, 
do  not  change  their  form,  and  frequently  only  persist 
lor  a  short  time,  soon  falling  away.  All  these  are 
classed  together  as  bud-scales  ;  they  really  represent 
only  the  bases  of  leaves  (Fig.  18  C). 

As  growth  goes  on  other  differences  appear.  The 
internodes  between  the  bud-scales  do  not  elongate,  so 


THE  SHOOT 


49 


that  while  the  scales  persist  the  young  shoot  seems  sur- 
rounded by  a  number  of  small  leaves  at  its  base  (Fig. 
1 8  C).  When  the  bud-scales  fall  off,  they  leave  the  base 
of  the  shoot  surrounded  by  scars,  which  mark  the  places 
of  their  original  attachment.  At  the  close  of  growth  on  the 
onset  of  winter,  the 
shoot,  now  become 
what  is  technically 
called  a  twig  (Fig. 
19),  shows  these 
scars  closely  placed 
together  round  its 
base.  In  the  winter 
it  is  easy  to  recog- 
nise the  amount  of 
growth  of  a  twig 
during  the  preced- 
ing year,  by  noting 
the  distance  be- 
tween this  collec- 
tion of  scars  and 
the  apex.  In  an 
older  twig  or  young 
branch  several 
such  collections  of 
scars  can  be  de- 
tected, and  so  the 
limits  of  each  year's  growth  can  be  easily  ascertained. 
With  the  opening  of  the  bud  and  the  expansion  of  its 
leaves  as  its  stem  elongates  we  can  trace  the  sensitive- 
ness of  the  shoot  to  the  various  influences  that  sur- 
round it.  We  have  pointed  out  the  response  its  axis 
makes  to  the  influence  of  gravity  and  lateral  light.  We 
have  also  incidently  mentioned  the  change  of  the  cur- 
vature of  the  leaves  which  sets  in  as  soon  as  the  bud 
opens.  The  change  is  a  response  to  the  access  of  light 

D 


FIG.  19.  Twig  of  3  years'  growth,  bs,  scars 
of  bud-scales  of  each  year.  The  twig  shows 
racemose  branching.  (After  Ward.) 


50  BOTANY 

which  accompanies  this  opening.  The  water  in  the  cells 
of  the  leaf  was  in  the  early  stages  of  development  dis- 
tributed mainly  to  those  of  the  under  side,  making  them 
most  turgid  and  causing  them  to  grow  most  freely. 
The  access  of  light  disturbs  this  relationship  and  the 
cells  of  the  upper  side  become  most  turgid;  the  conse- 
quent growth  causes  them  to  lose  the  concavity  of  their 
upper  sides;  they  become  flat  or  sometimes  slightly 
curved  in  the  other  direction. 

If  light  is  not  allowed  access  to  them  this  growth  of 
the  upper  side  is  very  much  interfered  with,  and  the 
leaves  show  but  little  change  of  curvature,  lying  close 
to  the  stem  as  it  elongates,  and  in  some  cases  not 
becoming  even  flat. 


CHAPTER  VI 

THE   CONSTRUCTION   OF   THE   SHOOT   SYSTEM 

THE  behaviour  of  the  plumule  in  elongating  and  ex- 
panding gives  rise  to  the  primary  shoot.  Every  succes- 
sive bud  and  branch  which  spring  from  it  increases  its 
dimensions  by  multiplying  the  number  of  twigs  it  bears. 
As  the  number  of  such  buds  upon  each  twig  is  fairly 
large,  we  see  that  the  young  branches  increase  in  a  kind 
of  geometrical  progression,  causing  the  formation  of  a 
large  shoot  system,  which  constitutes  the  body  of  a 
shrub  or  the  head  of  a  tree.  We  must  next  study  the 
construction  of  such  a  head. 

To  understand  it  we  must  inquire  what  are  the 
purposes  for  which  it  exists,  and  what  are  the  dangers 
against  which  it  must  protect  itself. 

We  have  already  drawn  attention  to  the  fact  that  the 
functions  of  the  root  and  the  shoot  are  fundamentally 
different.  That  being  so,  it  seems  clear  that  the  mode 
of  arrangement  of  the  parts  of  the  one  need  form  no 


CONSTRUCTION  OF  THE  SHOOT  SYSTEM     51 

rule  for  the  other.  There  is  nevertheless  a  general 
agreement  between  the  two,  though  careful  observa- 
tion shows  that  similarity  of  arrangement  subserves 
very  different  purposes.  The  arrangements  of  the  shoot 
all  bear  a  certain  relationship  to  life  in  air  and  its  conse- 
quent requirements,  and  show  further  a  co-ordination 
with  the  needs  which  are  cared  for  by  the  roots. 

We  have  seen  that  one  of  the  primary  objects  of  the 
latter  is  to  secure  a  firm  anchorage  for  the  plant  that  it 
may  be  able  to  maintain  its  erect  position.  The  de- 
velopment of  a  large  head  or  upper  part  makes  against 
such  anchorage,  by  offering  a  large  area  to  the  pressure 
of  wind  and  the  beating  of  rain — forces  likely  to  lead 
to  uprooting  from  the  soil. 

We  may  ask  why  such  a  risk  should  be  undertaken — 
why  the  sub-aerial  portion  of  the  plant  need  attain  the 
large  dimensions  it  possesses.  What  are  the  advantages 
which  are  afforded  by  a  widespreading  head  rearing 
itself  into  the  air?  Are  they  commensurate  with  the 
risk,  and  what  are  the  precautions  which  protect  the 
plant  in  face  of  the  dangers  it  involves  ? 

In  seeking  answers  to  these  questions  we  must  look  a 
little  more  closely  at  the  chief  features  of  the  upper 
portions  of  the  shoot  system.  We  soon  see  that  one  of 
the  objects  secured  by  the  method  of  development  which 
it  follows  is  the  great  amount  of  surface  in  proportion  to 
bulk  which  the  shoot  presents.  The  twigs  are  thin,  the 
leaves  flat.  We  have  indeed,  as  we  have  in  the  root, 
and  as  we  notice  in  the  case  of  the  large  seaweeds,  the 
bringing  of  the  structure  of  the  plant  into  relationship 
with  as  large  a  portion  of  the  environment  as  possible. 
Here  is  clearly  an  indication  or  suggestion  of  an  inter- 
change of  material  between  the  two. 

We  have  already  assumed  that  there  is  such  an  inter- 
change, and  may  now  examine  more  closely  its  nature. 
A  few  simple  observations  will  enable  us  to  prove  it. 


52  BOTANY 

Let  us  remove  a  twig  with  its  expanded  leaves  to  a  con- 
fined space,  so  that  we  examine  the  conditions  of  the  air 
around  it  to  see  what  changes,  if  any,  take  place.  Let 
us  shut  it  up  in  a  well-dried  bottle  and  keep  it  at  its 
accustomed  temperature.  We  shall  find  after  a  short 
time  that  the  sides  of  the  bottle  become  bedewed  with 
moisture,  and  a  little  later  we  shall  see  that  the  leaves 
upon  the  twig  and  at  least  its  upper  part  become  wilted 
and  drooping.  Part  of  the  work  of  the  shoot  is  clearly 
to  exhale  watery  vapour  from  its  surface. 


FIG.  20.  Section  of  leaf  showing  intercellular  spaces  and  stomata. 
The  cells  contain  chloroplasts.      X8o. 

If  careful  measurements  are  made  of  the  total  water 
a  plant  gives  off,  it  is  found  to  be  very  considerable  in 
amount,  and  to  be  given  off  during  the  whole  of  the  day 
in  quantities  varying  with  the  changing  conditions  sur- 
rounding the  plant.  The  structure  of  the  leaf,  to  which 
we  must  give  later  some  careful  attention,  shows  us 
that  the  intercellular  spaces  which  we  observed  in  the 
body  of  the  root  exist  in  even  greater  degree  in  the  leaf 
(Fig.  20)  and  yield  an  evaporating  surface  much  larger 
than  the  external  surface  of  the  twigs  and  leaves. 
These  internal  channels  communicate  with  the  exterior 
through  small  openings  in  the  limiting  membrane  of  the 


CONSTRUCTION  OF  THE  SHOOT  SYSTEM     53 

leaves  and  the  more  delicate  parts  of  the  twig.  These 
openings,  which  are  known  as  stomata,  are  themselves 
co-ordinated  with  the  regulation  of  this  exhalation  of 
vapour,  the  width  of  the  opening  being  capable  of 
variation  according  to  different  conditions.  We  must 
associate  the  evaporation  of  so  much  vapour  by  the 
leaves  with  the  very  large  absorption  of  water  we 
observed  in  the  root,  and  we  can  see  that  the  structure 
of  the  leaf  is  as  well  adapted  to  evaporation  as  that  of 
the  root  is  to  absorption.  Further  structural  adapta- 
tions to  this  maintenance  of  a  stream  of  water  through 
the  plant  will  become  evident  later,  but  in  the  mean- 
time we  can  see  in  the  features  already  alluded  to  a 
definite  relation  to  this  particular  interchange  between 
the  plant  and  its  surroundings  or  environment. 

Still  pursuing  our  inquiry,  we  may  notice  that  while 
the  general  colour  of  the  shoot  is  green,  the  depth  of  the 
green  tint  is  not  uniform.  The  flattened  parts  or  leaves 
are  of  a  brighter  green  than  the  cylindrical  axes,  and  in 
general  it  soon  appears  that  the  more  exposed  any  part 
of  the  shoot  is  in  its  young  and  most  delicate  condition, 
the  more  prominent  the  green  colour  becomes.  We 
have  consequently  a  suggestion  of  some  co-ordination 
between  exposure  and  colour.  Comparing  two  shoots 
growing  in  different  places  we  can  soon  associate  the 
optimum  brightness  with  the  best  illumination,  and  we 
are  led  to  infer  that  one  reason  for  the  flatness  of  certain 
parts  of  the  shoot  is  the  desirability  of  exposing  as  much 
of  their  surface  as  possible  to  light. 

We  have  already  called  attention  to  the  fact  that  the 
greater  part  of  the  plant's  food  is  manufactured  in  the 
leaves,  and  that  the  green  colouring  matter — chlorophyll 
— is  chiefly  concerned  in  making  it.  The  chlorophyll  is 
not  diffused  throughout  the  living  substance,  but  is 
confined  to  a  number  of  small  ovoid  bodies  which  are 
embedded  in  it,  and  these  green  bodies  are  placed  very 


54  BOTANY 

little  below  the  surface  of  the  leaves,  being  thus  covered 
only  by  a  thin  transparent  layer  of  cells.  The  dis- 
tribution of  these  green  bodies,  which  are  known  as 
chloroplasts,  so  bears  a  very  definite  relation  to  the 
incidence  of  the  light,  and  suggests  to  us  that  while  one 
duty  of  the  leaf  is  to  exhale  watery  vapour,  another 
is  to  secure  the  illumination  of  a  definite  part  of  its 
mechanism,  which  is  concerned  with  the  most  intimate 
questions  of  nutrition. 

As  the  two  functions  thus  suggested  are  found  upon 
further  inquiry  to  be  intimately  bound  up  with  the  well- 
being  of  the  plant,  we  must  examine  them  a  little  more 
closely  before  looking  for  the  ways  in  which  they  in- 
fluence the  form  and  position  of  the  shoot  system. 

There  are  two  reasons  for  the  copious  evaporation  of 
water  which  we  have  pointed  out.  The  first  is  con- 
nected with  the  problem  of  feeding,  as  we  noticed  in 
our  introductory  chapter.  Certain  constituents,  either 
entering  into  the  composition  of  the  food  itself  or 
necessary  factors  in  its  construction,  are  only  to  be 
found  in  the  soil  and  are  procured  therefrom  by  the 
roots.  These  compounds  are  absorbed  from  the  soil  in 
solution  in  the  water  entering  the  root  hairs,  and  the 
solutions  are  necessarily  very  dilute  to  facilitate  their 
passage  through  the  living  substance  of  the  hairs.  As 
with  a  rapidly-growing  plant  continuously  increasing 
quantities  of  these  substances  are  needed  for  nutritive 
purposes,  it  follows  that  large  quantities  of  the  solution 
must  be  absorbed.  In  the  plant  these  mineral  com- 
pounds are  taken  from  the  water,  and  the  great  bulk  of 
the  latter  is  evaporated  into  the  intercellular  passages 
and  the  vapour  subsequently  passed  out  of  the  stomata. 
Hence,  speaking  broadly,  the  more  water  that  is  taken 
up  and  subsequently  evaporated,  the  more  mineral 
matter  is  secured  for  the  use  of  the  organism. 

But  there  is  another  and  equally  important  function 


CONSTRUCTION  OF  THE  SHOOT  SYSTEM     55 

that  this  evaporation  discharges.  In  the  time  of  sun-, 
shine  a  great  deal  of  the  sun's  energy  in  the  form  of  heat 
and  light  is  falling  on  the  plant.  It  has  been  computed 
that  the  amount  is  so  great  that  it  would  raise  its 
temperature  to  such  a  dangerous  extent  that  if  no 
counter-influence  were  at  work  it  must  speedily  perish. 
Now  the  evaporation  of  water  always  requires  the 
expenditure  of  a  considerable  amount  of  heat,  and  we 
find  that  the  greater  part  of  the  heat  reaching  the 
plant  from  the  sun  is  devoted  to  the  vaporisation  of 
the  water  in  the  intercellular  passages  of  the  leaves  and 
other  parts.  The  normal  temperature  of  the  plant  is 
thus  maintained  in  the  face  of  the  enormous  absorption 
of  solar  heat  which  its  exposed  and  often  unprotected 
position  renders  inevitable. 

The  study  of  the  behaviour  of  the  chloroplasts  shows 
us  that  their  position  is  definitely  associated  with  the 
duty  which  we  have  attributed  to  them.  Not  only  is 
their  colour  dependent  on  their  exposure  to  light,  but 
the  part  they  play  in  the  construction  of  food  is  equally 
related  to  the  illumination  they  receive.  We  have 
already  spoken  of  the  work  done  by  the  chloroplasts, 
and  have  seen  that  they  construct  organic  food  in  the 
form  of  sugar  and  .similar  compounds  from  the  carbon 
dioxide  of  the  air,  together  with  a  portion  of  the  water 
supplied  them  by  the  root.  Carbon  dioxide  is  present 
in  very  small  proportion  in  the  air,  only  some  3  or  4 
parts  in  10,000.  The  construction  of  food  from  such 
antecedents  is  only  possible  in  the  presence  of  light; 
two  things  therefore  must  be  secured — a  wide  surface 
and  preferably  a  copiously  subdivided  one,  to  bring  as 
much  air  as  possible  into  contact  with  it,  and  as  com- 
plete an  exposure  as  possible  of  the  chloroplasts  to 
light  to  enable  the  construction  of  the  sugar  to  go  on. 

The  form  and  disposition  of  the  shoot  system  must 
be  regarded  from  the  point  of  view  of  these  require- 


56  BOTANY 

ments.  True,  at  first  sight  they  seem  a  little  antagonistic 
to  other  needs.  The  evaporation  of  the  water  and  the 
illumination  of  the  chloroplasts  demand  a  large  and 
increasing  shoot-body,  but  its  increase  in  size  brings 
with  it  a  distinct  danger  to  the  stability  which  we  have 
seen  is  one  of  the  first  necessities  of  the  plant  as  a  whole. 
The  reconciling  of  these  demands  must  add  to  the 
interest  with  which  we  study  the  form  and  distribution 
of  the  members  of  the  shoot  system. 

We  have  seen  that  the  axis  of  the  latter  is  very  much 
subdivided,  the  ultimate  divisions,  the  branches,  taper- 
ing to  points,  in  some  cases  extremely  gradually,  in 
others  more  abruptly.  These  cylindrical  or  conical 
divisions  bear  a  number  of  flattened  organs,  the  leaves, 
which  are  usually  attached  to  the  axis  by  flexible  stalks 
or  particles.  We  can  now  see  the  reason  for  this  sub- 
divided conformation.  It  secures  strength  by  the 
cylindrical  form  of  the  twigs,  surface  by  the  flattened 
form  of  the  leaves.  The  winds  can  blow  freely  through 
the  mass  of  twigs,  while  the  long  leaf  stalks  allow  of 
sufficient  displacement  of  the  flattened  parts  when  the 
pressure  of  the  wind  is  brought  to  bear  upon  them. 
Moreover,  the  parts  concerned  are  all  extremely  flexible 
and  elastic,  so  that  they  can  yield  to  pressure  and 
regain  their  positions  as  soon  as  it  is  removed. 

The  form  of  the  shoot  system  of  a  plant  will  depend 
upon  the  manner  of  its  branching,  and  the  number,  size, 
and  arrangement  of  the  leaves  its  branches  bear. 

The  branching  will  be  affected  by  two  main  factors : 
firstly,  the  number  of  branches  produced  at  a  node ; 
secondly,  the  relative  degree  of  growth  of  each  main 
branch  and  those  to  which  it  gives  rise. 

The  first  of  these  does  not  show  as  much  variety  as 
might  be  expected.  Usually  one  branch,  not  infre- 
quently two,  appear  at  a  node,  but  seldom  more. 

The   second   factor,    however,    plays    a   much   more 


CONSTRUCTION  OF  THE  SHOOT  SYSTEM     57 

prominent  part  in  the  construction  of  the  head.  If  the 
first  axis  grows  more  vigorously  than  its  branches — a 
behaviour  we  found  to  lead  to  the  formation  of  a  tap 
root  in  the  root  system — and  if  each  of  the  branches  in 
turn  is  longer  and  stronger  than  those  arising  from  it, 
the  ultimate  form  of  the  head  is  pyramidal,  for  the 
successive  branches  arise  nearer  and  nearer  to  the  apex, 
and  so  long  as  the  growth  is  regular  the  lowest  will  be 
generally  the  most  widespreading.  This  is  true  of  the 
series  of  branches  which  each  of  them  bears.  This  type 
of  branching  is  said  to  be  indefinite  or  racemose  (Fig.  19), 
and  it  is  illustrated  by  such  trees  as  the  spruce  fir. 

If,  on  the  other  hand,  the  growth  of  each  axis  or 
branch  is  soon  checked  and  so  its  development  becomes 
exceeded  by  the  growth  of  the  daughter-axes  to  which 
it  gives  rise,  the  head  will  be  sub-globular  or  rounded. 
The  exact  shape,  however,  will  largely  depend  on  the 
number  of  branches  springing  from  below  the  apex  of 
each  in  turn,  for  these  all  arise  at  the  same  node.  They 
are  therefore  on  the  same  level,  and  do  not  grow  in 
what  is  called  acropetal  succession,  as  in  the  first  case. 
A  very  common  form  is  that  in  which  each  branch  is 
solitary.  This  form  of  branching  is  called  definite  or 
cymose.  Examples  are  afforded  by  the  elms,  oaks,  and 
many  other  forest  trees  (Fig.  21). 

Another  factor  in  the  shape  which  the  branching 
helps  to  give  to  the  shoot  system  is  the  non-develop- 
ment of  some  of  the  buds.  We  have  seen  that  a  bud  is 
produced  in  the  axil  of  every  foliage  leaf.  It  often 
happens,  however,  that  a  twig  cannot  adequately  feed 
all  the  buds  it  bears.  Hence  some  perish  and  others 
remain  dormant  for  some  time,  circumstances  which 
cause  a  good  deal  of  irregularity. 

Before  we  study  the  influence  of  the  arrangement  of 
the  leaves  upon  the  form  of  the  shoot  and  the  shoot 
system  we  must  look  a  little  more  closely  at  the  peculi- 


58  BOTANY 

arities  of  their  flattened  form.  We  have  seen  wherein 
lie  its  advantages,  but  we  must  consider  also  the  diffi- 
culties and  even  dangers  which  it  involves.  Difficulties 
arise  from  the  certainty  that  the  leaves  must  encounter 
rough  weather  in  the  course  of  the  year.  Rain  may 
soak  them  through,  wind  may  tear  them  apart,  or  even 
strip  them  from  the  twigs.  How  are  these  perils  met  ? 


FIG.  21.  Diagram  of  forms  of  cymose  branching. 

There  are  two  reasons  why  rain  falling  upon  them 
does  not  affect  them  seriously  for  a  long  time.  Gener- 
ally the  shape  of  each  is  such  that  there  is  a  longitudinal 
groove  all  along  its  upper  surface,  running  from  apex  to 
base  in  the  centre  of  the  flattened  blades.  This  con- 
ducts the  water  away  as  fast  as  it  falls  upon  the  leaf, 
either  towards  the  apex  or  towards  the  base.  In  the 
latter  case  the  groove  is  continued  along  the  leaf  stalk 
so  that  the  water  is  taken  to  the  ground.  The  second 
reason  is  that  the  outer  layers  of  the  walls  of  the  cells 


CONSTRUCTION  OF  THE  SHOOT  SYSTEM     59 


FIG.    22.    Venation   of 
leaf. 


of  the  upper  surface  become  almost  impermeable  by 
water.  It  is  only  after  long  soaking,  therefore,  that 
any  can  find  entrance. 

The  danger  from  wind  is  perhaps  greater  than  that 
from  rain.  The  leaf-blade,  however, 
though  delicate  and  thin,  is  never- 
theless very  strong  and  not  easily 
torn.  Running  through  it  are  the 
ultimate  endings  of  the  vascular 
strands  we  have  already  noted  in  the 
root,  the  conducting  tissue  (Fig.  22). 
These  strands  form  the  so-called  veins 
of  the  leaf  and  they  constitute  a  net- 
work of  very  tough  fibrous  bands 
upon  which  the  delicate  tearable 
tissue  is  supported.  They  generally 
strengthen  particularly  the  margins 
and  the  apex  of  the  leaf-blade  and  protect  it  from  being 
torn.  The  blade,  therefore,  when  acted  on  by  wind  is 
seldom  either  bent  or  curled,  but  is  made  to  play  as  a 
single  rigid  piece  moving  up  and  down  without  losing  its 
flatness  for  a  moment. 

The  danger  of  stripping  from  the  twig  is  dealt 
with  differently.  When  the  plant  is  of  a  sturdy,  rigid 
habit,  the  leaves  are  usually  attached  to  the  stem  very 
strongly,  and  are  bent  upwards  so  that  the  direction 
of  the  wind  must  drive  them  towards  the  stem,  and  its 
force  cannot  be  felt  between  the  latter  and  the  leaf's 
upper  surface.  More  frequently,  however,  we  find  that 
the  blade  of  the  leaf  is  attached  to  the  twig  by  means  of 
a  tough,  flexible  stalk,  capable  of  movement  in  almost 
every  direction  on  its  point  of  attachment.  The  elasti- 
city is  so  great  and  so  readily  called  into  play  that  with 
even  the  lightest  breezes  the  leaves  of  most  trees  are 
seen  to  swing  to  and  fro  with  the  greatest  freedom. 

The  form  of  the  head  of  the  tree  is  influenced  by  the 


6o 


BOTANY 


shape  as  well  as  the  arrangement  of  the  leaves.  Usually 
leaves  consist  of  three  regions,  a  flattened  part  or  blade, 
a  leaf-stalk  or  petiole,  and  a  leaf  base  by  which  it  is 
attached  to  the  stem.  If  we  regard  it  as  an  outgrowth 
from  the  stem,  we  find  that  it  assumes 
its  flattened  form  by  developing  a  wing 
on  each  side,  the  outgrowth  itself  also 
becoming  flat.  If  the  outgrowth 
branches  and  only  its  branches  develop 
wings  we  have  what  is  commonly 
termed  a  compound  leaf  (Fig.  23). 

The  leaf-stalk  is  the  lower  part  of  the 
axis  of  the  leaf  and  it  is  continued 
forwards  to  the  tip,  the  part  which  has 
become  winged  being  called  the  mid-rib. 
In  some  cases  the  whole  of  the  axis  of 
the  leaf  becomes  winged.  The  leaf  is 
said  then  to  be  sessile  or  to  have  no 

stalk>      M  ^  bage  of  the  kaf  are  yery 

frequently  two  small  outgrowths,  of  the 
nature  of  leaf  branches.  These  are  known  as  stipules. 
They  vary  a  good  deal  in  shape  and  size. 

The  object  aimed  at  in  the  distribution  of  leaves  on  a 
tree  is  the  covering  of  the  framework  of  its  head  as  com- 
pletely as  possible  by  a  thin  curtain  of  leaves,  as  free 
from  unoccupied  gaps  as  possible  ;  the  leaves  themselves 
must  be  so  arranged  that  little  shading  of  one  part  by 
another  shall  occur.  If  we  stand  under  a  tree  and  look 
up  through  its  branches  we  find  the  leaves  are  not  dis- 
tributed all  about  the  interior  of  the  space  occupied  by 
the  boughs  and  branches  ;  they  are  seen  to  be  a  more  or 
less  complete  covering  to  the  head.  In  a  humbler  type, 
such  as  a  thistle  or  a  sunflower,  the  leaves  overlap  very 
little,  so  that  practically  the  whole  leaf-surface  is  ex- 
posed to  the  light  during  at  any  rate  some  part  of  the  day. 

The  leaves  are  arranged  in  various  ways  upon  the 


FlG'  ^Compound 


CONSTRUCTION  OF  THE  SHOOT  SYSTEM     6r 

stem,  but  always  occur  in  vertical  or  nearly  vertical 
rows.  Sometimes  only  one  leaf  originates  at  each  node, 
sometimes  two,  or  occasionally  more.  When  only  one 
occurs  the  leaves  are  found  to  be  arranged  spirally  or 
alternately  up  the  stem.  When  more  than  one  there  is 
said  to  be  a  whorl  of  leaves  at  each  node.  Frequently 
the  whorl  consists  of  two  leaves  only.  Successive 
whorls,  whatever  the  number  of  leaves,  have  their 
separate  leaves  placed  opposite  the  spaces  between 
the  leaves  of  the  whorls  above  and  below  them. 

The  number  of  the  vertical  rows  is  correlated  with  the 
shape  and  size  of  the  leaves  which  compose  them. 
Leaves  with  very  broad  bases,  often  indented,  and 
tapering  fairly  rapidly  to  a  pointed  apex,  known  techni- 
cally as  ovate  or  cordate  leaves,  generally  occur  opposite 
to  one  another  on  the  stem,  there  being  only  two  rows. 
Sometimes  they  have  short  stalks,  sometimes  none. 
When  the  leaf  has  its  broadest  part  near  the  middle  and 
tapers  to  both  apex  and  base  it  is  termed  an  oval  or  ellip- 
tical leaf;  such  are  generally  arranged  in  three  rows. 
When  still  narrower,  becoming  what  are  known  as 
lanceolate  leaves,  the  number  of  rows  increases  to  five  or 
eight.  Still  narrower  leaves  occur  in  greater  numbers  of 
ranks  still.  We  see  thus  a  co-ordination  between  the 
shapes  and  dimensions  of  the  leaves  and  their  mode  of 
attachment  to  the  stem,  just  such  a  co-ordination  as  we 
should  expect  when  we  remember  the  disadvantages 
which  would  arise  from  a  crowding  together  of  large 
ovate  leaves  in  several  ranks,  or  the  sparse  scattering  of 
linear  or  narrow  leaves  in  few  rows. 

When  we  study  in  this  way  the  shoot  systems  of 
different  plants  we  find  them  to  be  in  harmony  with 
their  surroundings  as  fully  as  are  the  root  systems.  The 
surroundings  influence  the  plant  very  forcibly  while  it 
is  developing,  and  many  of  the  results  of  its  development 
can  only  be  understood  by  observing  that  they  are 


62  BOTANY 

essentially  purposeful.  The  only  mode  of  securing  this 
adjustment  with  the  environment  which  is  possible  is 
that  of  regulating  its  growth. 

During  the  early  development  and  growth  the  plant 
exhibits  in  its  shoots  as  in  its  roots  powers  of  purposeful 
response  to  certain  features  of  the  environment  which  it 
is  capable  of  appreciating.  If  we  examine  the  plumule 
or  young  bud  of  the  seedling  as  soon  as  it  begins  to  grow, 
we  shall  notice  the  same  perception  of  direction  as  we 
observed  in  the  root.  As  the  latter  would  persist  in 
growing  downwards,  curving  itself  if  its  apex  pointed 
in  any  other  direction,  so  the  shoot  persists  in  growing 
upwards.  The  sensitive  part  is  not  so  easy  to  localise 
as  in  the  root,  but  careful  experiments  made  on  various 
plants  have  proved  that  the  perceptive  part  of  the 
shoot  is  the  tip  and  that  the  sensitive  zone  does  not 
extend  far  downwards.  The  response  to  the  stimulus  is 
brought  about  in  the  same  way  in  the  two  cases,  viz.,  by 
a  modification  of  the  growth,  and  it  is  clearly  purposeful, 
to  plant  the  root  in  the  earth  and  the  shoot  in  the  air. 
There  is  a  close  resemblance  again  in  their  behaviour 
between  the  primary  branches  of  the  stem  and  root. 
None  of  them  grows  in  the  same  direction  as  the  axis 
from  which  it  springs,  but  usually  they  stretch  out  nearly 
at  right  angles  to  it.  This  is  a  response  to  the  stimulation 
of  gravitation  in  both. 

Another  factor  which  is  of  much  greater  importance 
in  the  case  of  the  shoot  than  in  that  of  the  root  is  the 
incidence  of  lateral  light,  which  helps  to  determine  the 
position  of  the  branches  as  well  as  of  the  leaves.  If  the 
light  falls  on  a  shoot  more  intensely  on  one  side  than 
another,  the  rate  of  growth  very  speedily  changes  so  as 
to  cause  the  growing  region  of  the  stem  to  bend  or  curve 
till  its  apex  is  directed  towards  the  point  from  which  the 
strongest  light  is  coming.1  The  plant  exhibits  a  power 

1  A  figure  illustrating  this  is  given  in  the  Biology  primer,  p.  71. 


THE  STRUCTURE  OF  THE  SHOOT    63 

of  perceiving  or  appreciating  differences  of  intensity  o^ 
illumination.  This  sensitiveness  is  of  the  greatest  value 
io  the  shoot,  for  as  the  stem  bends  towards  the  source  of 
the  light  the  leaves  which  are  expanded  nearly  at  right 
angles  to  it  are  exposed  to  the  rays  which  they  need  for 
the  manufacture  of  sugar. 

The  leaves  also  manifest  an  independent  sensitiveness 
to  light.  They  are  generally  so  expanded  as  to  expose 
their  upper  surfaces  to  the  sunshine.  If  this  position 
•cannot  be  attained  without  a  movement  of  the  leaf  this 
movement  is  effected  and  supplements  the  other.  The 
leaf-blade  twists  on  its  petiole,  or  the  petiole  twists  in 
such  a  way  as  to  expose  the  surface  of  the  blade. 

With  the  same  sensitiveness  to  light  we  see  thus  that 
the  different  members  of  the  plant  respond  differently, 
but  always  purposefully,  to  it.  The  root  grows  away 
from  the  incident  rays,  penetrating  into  the  deeper 
crevices  of  the  soil ;  the  stem  grows  towards  them, 
while  the  leaf  places  itself  across  their  path. 

The  positions  assumed  by  the  stem,  branches,  and 
leaves  are  greatly  influenced  by  the  various  stimulations 
they  receive ;  some  respond  more  actively  to  one,  others 
to  another;  but  all  show  both  perception  and  response 
as  they  adapt  themselves  to  their  environment. 


CHAPTER  VII 

THE    STRUCTURE   OF   THE    SHOOT 

WE  must  now  examine  what  are  the  arrangements  of 
the  internal  structures  of  the  shoot  which  enable  it  to 
carry  on  these  duties.  Though  the  shoot  is  to  be 
regarded  as  a  single  system  comparable  with  the  root, 
the  duties  discharged  by  its  cylindrical  and  its  flattened 
parts  are  so  far  distinct  that  it  will  be  well  to  consider 
them  separately  from  our  present  point  of  view. 


64  BOTANY 

When  we  examine  the  plumule  we  find  it  to  be  com- 
posed of  cells  resembling  those  of  the  radicle.  They  are 
at  first  all  alike,  and  only  slowly  do  differences  become 
apparent.  At  the  apex  we  find  them  meristematic, 
that  is,  each  cell  has  the  power  to  divide  into  two.  A 
little  farther  back  they  increase  in  size  and  become 
vacuolated.  If  we  take  a  longitudinal  section  at  this. 
d  pe  pi  a£e>  we  fi11^  that,  as  in  the 

root,  we  can  distinguish 
three  regions  which  are 
faintlY  indicated  (Fig.  24). 
The  central  strand  or 
plerome  is  visible,  appear- 
ing conical  in  shape  as  in 
the  root.  Outside  it  lies 
a  periblem,  and  this  is 
covered  by  a  dermatogen, 
a  layer  of  a  single  cell  in 
thickness.  These  two  are 
not  conical,  but  are  thrown 
into  irregularity  by  the 
outgrowth  of  the  leaves. 
The  leaves  and  branches 
FIG.  24.  Growing  point  of  stem  of  differ  in  their  origin  from 

Dicotyledon,     d,  dermatogen ;  pe,    the  branches  of  the  root  as 

young  they begin  with  the  Ollt" 

growth  of  the  periblem, 
which  pushes  the  dermatogen  before  it.  The  plerome 
takes  no  part  in  their  formation.  As  the  plumule  gets 
older  its  elongation  proceeds  by  the  continued  formation 
of  new  cells  and  their  subsequent  growth.  This  goes  on 
for  some  time,  and  extends  as  a  rule  further  back  than  it 
does  in  the  root.  The  growing  region  is  a  little  more 
complex  in  the  stem  than  in  the  root,  because  the  cells 
do  not  all  grow  alike,  those  of  the  nodes,  or  places 
.where  the  leaves  arise,  elongating  scarcely  at  all,  while 


THE  STRUCTURE  OF  THE  SHOOT 


those  of  the  internodes  are  very  vigorous.  The  leaves 
on  the  nodes  elongate  from  the  first,  but  the  branches 
in  their  axils  appear  much  later. 

As  the  seedling  grows,  it  prepares  for  the  discharge  of 
the  duties  which  devolve  upon  it.  What  we  are  about 
to  describe  of  its  structure  corresponds  almost  exactly 
with  the  structure  of  each  year's 
twigs  of  the  tree  or  shrub  into 
which  ultimately  it  develops. 

The  two  main  duties  of  the 
stem  we  have  seen  to  be  the 
support  of  the  head  or  leaf-bear- 
ing part  of  the  shoot  and  the 
transport  of  the  water  and 
mineral  compounds  absorbed 
by  the  roots  to  the  seat  of  con- 
struction of  organic  substance. 
Both  these  objects  are  carried 
out  by  the  arrangements  in  the 
central  cylinder,  and  both  de- 
pend upon  the  development  of 
vascular  strands  connected  with 
those  of  the  root.  If  we  look 
at  a  longitudinal  section  of  a 
whole  plant  we  find  that  these  FlG- 
strands  are  continuous  through- 
out it  though  they  are  arranged 
differently  in  its  different  regions.  In  the  root  we  found 
the  strands  of  wood  lying  sometimes  separately  in  a 
central  ring,  sometimes  joined  to  form  a  solid  cone  of 
wood.  Other  strands,  soft  in  nature,  known  as  bast,  lie 
between  them  or  between  their  outer  limbs  when  they 
are  fused  in  the  centre.  As  we  pass  upwards  we  find 
that  in  the  region  just  below  the  cotyledons  a  certain 
rearrangement  of  the  strands  takes  place.  The  bundles 
shift  their  relative  positions  and  the  wood  strands  come 


25.  Diagram  showing 
the  general  structure  of  a 
dicotyledonous  plant. 


66  BOTANY 

to  lie  exactly  inside  the  bast  strands,  the  two  being 
separated  only  by  a  layer  of  meristematic  cells  known  as 
cambium.  The  bundles  in  the  shoot  are  known  as  con- 
joint bundles  from  this  association  of  the  wood  and  bast. 
The  wood  strands,  further,  become  twisted  on  their  long 
axes  in  this  same  region,  so  that  the  protoxylem,  which  in 
the  root  is  on  the  outside,  is  in  the  stem  on  the  inner  face. 
The  bundles  are  wedge-shaped  much  as  they  are  in  the 
root.  Each  seems  thus  to  have  turned  completely  round 
so  as  to  face  in  the  opposite  direction.  Instead  of  the 
cylinder  being  solid  in  the  centre,  the  conjoint  strands 
always  stand  round  its  periphery  so  that  there  is  a  large 
unoccupied  space  in  the  centre,  known  as  the  pith. 

Following  them  to  the  growing  end  of  the  stem  we 
find  that  they  do  not  terminate  in  its  growing  cells,  but 
can  be  traced  into  the  leaves  through  their  petioles. 
In  the  latter  they  usually  form  a  half  cylinder  open  on 
the  upper  side,  instead  of  a  complete  hollow  cylinder  as 
in  the  stem.  From  the  petiole  they  can  be  traced  into 
the  flattened  portion  of  the  leaf,  where  they  form  the 
network  which  we  call  the  veins  of  the  leaf. 

While  the  leaf  and  stem  are  very  young  we  find  in 
them  the  first  traces  of  the  origination  of  these  strands. 
They  appear  in  the  growing  point  a  little  way  back, 
as  separate  strands  in  the  plerome,  made  up  of  small 
cells,  longer  than  broad,  defined  from  the  rest  chiefly  by 
their  smaller  transverse  diameters.  They  are  all  meri- 
stematic and  only  slowly  lose  the  power  of  dividing.  A 
transverse  section  (Fig.  26)  of  the  plerome  shows  these 
little  strands  as  wedge-shaped  areas,  the  procambial 
strands,  arranged  in  a  circle  near  the  outside  of  the 
plerome,  separated  by  narrow  areas  known  as  medullary 
rays.  As  they  get  older  the  cells  become  changed  into 
their  adult  form.  The  change  in  the  wood  cells  is  asso- 
ciated with  growth  in  diameter  and  irregular  thickening 
of  the  walls,  making  them  appear  as  if  marked  out  into 


THE  STRUCTURE  OF  THE  SHOOT    67 


FIG.  26.  Diagram  of  sections  of  stem  of  dicotyledon  at  three  ages. 
A,  young  condition,  showing  commencement  of  differentiation  of 
the  plerome  and  its  vascular  strands:  a,  strand;  b,  limits  of  the 
plerome;  c,  periblem;  m.r.,  medullary  ray;  pi,  pith.  B,  a  little 
older  stage:  p,  bast;  x,  wood;  c,  cambium;  i.e.,  interfascicular  cam- 
bium: (one  of  the  strands  has  been  shaded).  C,  older  stage,  after  the 
commencement  of  secondary  thickening:  px,  protoxylem  or  first- 
formed  wood;  x,  secondary  wood;  ph,  secondary  bast.  (After  Sachs.) 


68  BOTANY 

curious  patterns ;  with  substitution  of  lignin  for  cellu- 
lose as  the  material  of  which  they  are  composed;  and 
with  the  disappearance  of  many  of  the  transverse 
separating  walls,  causing  a  vertical  row  of  cells  to  be- 
come a  vessel.  The  cells  to  show  the  change  first  are 
those  on  the  inside  of  the  wedge-shaped  strand — the 
protoxylem.  In  these  the  thickening  of  the  walls  is 
laid  down  in  the  shape  of  a  spiral  band,  or  a  series  of 
rings.  These  vessels  remain  of  small  diameter.  The 
other  wood  cells  and  vessels  are  thickened  more  irre- 
gularly and  are  called  reticulated  ;  in  some  cases  when 
the  thickening  deposit  leaves  only  very  small  thin  spots 
they  are  known  as  pitted  elements  (Fig.  13,  p.  35). 

The  bast  of  the  strand  begins  to  be  differentiated  on 
the  side  nearest  the  periphery,  where  the  cells  are  called 
the  protophloem.  The  vessels  of  the  bast  are  sieve  tubes 
(Fig.  27)  as  in  the  root.  The  other  elements  are  mainly 
elongated  cells  with  thin  cellulose  walls. 

As  the  differentiation  begins  at  the  front  and  back  of 
the  bundle  and  advances  in  each  direction  the  wood  and 
bast  are  not  very  long  in  meeting.  In  plants  that  only 
live  for  a  few  weeks  or  months  they  come  into  actual 
contact,  but  to  those  whose  lives  are  longer  provision  is 
made  for  further  development  by  the  last  layer  left 
between  them  remaining  meristematic  or  capable  of  con- 
tinuous dividing.  This  is  the  cambium  layer  of  which 
we  have  spoken.  It  is  only  a  single  cell  in  thickness. 

This  arrangement  of  the  supporting  tissue  is  very 
strong  and  most  economical.  The  hollow  cylinder  or 
tube  is  one  of  the  strongest  forms  of  support  that  a 
structure  can  possess.  It  has,  too,  a  certain  flexibility, 
for  while  the  strands  are  gradually  hardening  they  can 
bend  freely  without  breaking.  The  young  stem  thus 
shows  itself  built  for  toughness  and  elasticity,  so  pos- 
sessing a  power  of  bending  to  wind  and  recovering  as 
the  force  of  the  air  passes  it.  The  continuity  of  the 


THE  STRUCTURE  OF  THE  SHOOT 


69 


vascular  strands  throughout  the  plant  ensures  the  proper 
distribution  of  the  water  absorbed  from  the  soil. 

Certain  other  features  of  the  framework  of  the  plant 
next  call  for  attention.  If  we  examine  the  outer  layer, 
called  in  the  stem  and  leaf  the  epidermis,  we  find  it  as  a 
continuous  sheet  over  the  whole,  and  in  most  cases  a 


FIG.  27.  Sieve  tube  from  stem  of  Cucurbita.  A,  transverse;  B,  longitu- 
dinal section ;  s.p.,  sieve  plate ;  c,  companion  cell.  (After  Strasburger.) 
X5oo. 

single  cell  in  thickness.  A  delicate  structure  like  a  seed- 
ling, whose  cells  are  filled  with  water,  is  exposed  to  a 
general  evaporation  at  the  surface.  This,  if  not  guarded 
against,  would  lead  to  a  loss  of  water  beyond  the  control 
of  the  plant,  and  would  interfere  with  the  proper  con- 
duction of  the  water  to  the  places  where  the  construction 
of  sugar  takes  place.  We  find  a  very  simple  but  very 
effective  protective  mechanism.  The  outer  walls  of  the 
cells  of  the  epidermis  become  thickened  and  their 
external  layers  are  changed  into  a  very  impermeable 


70  BOTANY 

material  called  cutin.  These  external  layers  can  be 
stripped  off  from  large  pieces  of  the  surface  in  a  kind  of 
pellicle,  which  is  known  as  the  cuticle.  It  is  developed 
more  freely  over  the  leaves  than  over  the  stem. 

This  layer  serves  too  as  a  protection  against  cold. 
For  this  purpose  many  plants  have  an  additional  safe- 
guard, in  the  shape  of  hairs,  or  outgrowths  of  the 
epidermal  cells,  forming  a  fine  felt  work  over  their  sur- 
faces, clothing  them  indeed  in  a  kind  of  cotton  garment. 

Both  cuticle  and  hairy  coating  serve  also  to  protect 
the  delicate  surface  from  injury  by  rain. 

The  outer  coating  or  cuticle,  covering  as  it  does  the 
whqle  exterior,  would  be  a  source  of  danger  to  the  plant 
by  preventing  almost  all  evaporation,  if  it  were  altogether 
intact.  The  epidermis  is  pierced  by  small  apertures, 
which  are  the  openings  of  the  system  of  intercellular 
spaces  or  passages  we  saw  to  be  developed  in  the  root 
and  which  we  now  find  to  extend  throughout  the  whole 
of  the  shoot  as  well.  These  stomata,  as  they  are  called, 
are  more  numerous  in  the  leaf  than  in  the  stem,  but  they 
are  present  in  the  latter  so  long  as  it  is  young.  The 
aperture  or  stoma  is  surrounded  by  two  cells  called 
guard-cells,  which  are  attached  together  at  their  ends 
but  not  in  their  centre.  They  are  kidney-shaped  in 
appearance,  and  when  filled  with  water  they  stretch  so 
as  to  draw  apart  in  the  centre,  opening  the  stoma  (Fig. 
28).  When  the  water  is  withdrawn  from  them  they  fall 
together  and  close  the  aperture.  This  arrangement 
thus  allows  the  necessary  evaporation  of  water  to  take 
place.  The  vapour  is  formed  in  the  intercellular 
passages  and  passes  out  through  the  stomata,  the 
width  of  the  apertures  being  regulated  by  the  amount  of 
water  in  the  guard-cells,  which  in  turn  depends  on  the 
amount  of  water  in  the  plant. 

The  layer  of  cells  between  the  central  cylinder  and 
the  epidermis,  which  is  the  continuation  backwards  of 


THE  STRUCTURE  OF  THE  SHOOT    71 

the   periblem,   is  the  cortex.      Its  composition  is  very 
varied  as  the  plants  grow  older.     In  the  young  condition 
it  is  only  noteworthy  because  its  outer  layers  of  cells 
contain  the  green  bodies  we  have  called  chloroplasts. 
The   great   development   of   branching   which   takes 


FIG.  28.  Epidermal  cells  of  leaf  showing  three  stomata  in 
various  stages  of  opening. 

place  necessitates  a  considerable  enlargement  of  this 
primary  structure.  The  increase  in  number  of  the 
leaves  makes  it  important  to  increase  the  means  of 
transport  of  water;  the  slender  cylindrical  tube  of  the 
young  stem  soon  becomes  unable  to  support  the  weight 
resulting  from  its  greater  size  and  the  number  of  its 
branches.  The  transport  of  food  to  its  different  parts 
makes  increasing  demands  upon  its  bast.  We  must 
examine  the  way  in  which  these  necessities  are  supplied. 


72  BOTANY 

As  the  stem  grows  we  find  additional  vascular  strands 
continually  being  developed,  a  change  directed  especially 
to  the  strengthening  of  the  stem,  as  the  new  strands  are 
not  directly  connected  with  the  leaves.  The  original 
strands  also  are  much  enlarged  and  strengthened. 

All  this  work  is  done  by  the  cambium  layer.  Part  of 
the  original  bundles,  as  we  have  seen,  consists  of  this 
tissue,  which  is  hence  called  fascicular  cambium.  By 
continual  division  of  its  cells,  mainly  in  a  direction 
parallel  with  the  outside  of  the  stem,  masses  of  cells  are 
produced  between  the  wood  and  the  bast.  One  layer  of 
these  remains  cambium — those  on  the  inside  of  it  are 
changed  into  wood,  those  on  its  outside  into  bast. 

Very  soon  after  this  process  has  been  set  up  in  the 
bundle  the  cells  which  lie  between  the  strands,  known  as 
the  medullary  rays,  are  the  seat  of  change.  No  doubt  the 
multiplication  of  the  cells  of  the  cambium  sets  up  a 
strain  in  the  ray  cells  adjoining  them,  stretching  them, 
or  dragging  upon  them.  The  stimulus  of  this  strain 
makes  certain  of  these  cells,  extending  across  the  ray, 
begin  to  divide  in  their  turn,  and  soon  the  rays  are  all 
crossed  by  layers  of  meristematic  cells,  joining  up  into 
a  ring  the  isolated  cambiums  of  the  bundles.  These 
new  portions,  which  complete  the  ring,  are  known  as 
inter  fascicular  cambium.  The  whole  ring  now  behaves 
as  the  original  cambium  of  the  bundles  does,  and  soon  a 
ring  of  wood  is  formed  in  front  of  and  a  ring  of  bast  behind 
it.  The  parts  of  the  ring  formed  by  the  interfascicular 
cambium  have  no  connection  directly  with  the  leaves. 

As  new  leaves  arise  at  the  apices  of  the  twigs  the 
vascular  strands  belonging  to  them  are  connected  with 
this  vascular  cylinder  as  were  the  primary  ones,  for  the 
structure  of  the  young  twigs  resembles  in  all  points  that 
which  we*  have  described  for  the  seedling. 

At  the  end  of  each  year,  in  our  climate,  the  growth  of 
most  trees  ceases,  owing  to  the  fact  that  the  leaves  fall 


THE  STRUCTURE  OF  THE  SHOOT 


73 


off.  When  it  is  resumed  on  the  putting  out  of  the 
leaves  of  the  next  year  this  process  recommences. 
There  is  a  difference  in  appearance  between  the  wood 


FK 


.  29.  Section  of  twig  of  lime-tree,  3  years  old.     ix,  2x, 
the  successive  annual  rings  of  wood.     (After  Kny.) 


formed  at  the  end  of  the  season  and  at  the  beginning,  so 
that  the  formation  of  each  year  stands  out  distinct  from 
that  of  the  next,  when  a  transverse  section  of  a  twig  or 
branch  is  examined.  Each  year's  formation  is  spoken 
of  as  an  annual  ring.  The  rings  of  wood  are  easily  seen, 
but  those  of  bast  are  not  so  conspicuous  (Fig.  29). 


74 


BOTANY 


When  a  tree  gets  old  the  central  part  of  its  wood 
usually  dies  and  becomes  very  hard.  The  only  living 
wood  is  a  narrow  area  close  to  the  cambium.  This  is 
known  as  the  sap-wood,  or  alburnum,  the  dead  centre 
being  the  heartwood  or  duramen. 

This  substitution  of  a  solid  core  for  a  hollow  cylinder 
of  wood  is  necessary  for  the  strengthening  of  the  trunk 
and  branches.  In  this  well-developed  form  the  mass  of 
the  body  of  both  shoot  and  root  is  made  up  of  hard  wood. 
As  the  trunk  and  branches  gradually  thicken,  their 
outer  regions  are  strengthened  by  the  production  of  bark. 
This  begins  in  the  stem  as  in  the  root  by  the  forma- 
tion of  sheets  of  cork.  In 
the  root  these  layers  arise 
in  the  pericycle ;  in  the  stem 
they  begin  in  the  cortex  just 
below  the  epidermis  (Fig. 
30).  As  the  years  pass, 
more  and  more  layers  of 
cork  are  formed  deeper  and  deeper  in  this  region.  The 
outer  ones  are  pierced  by  lenticels  (Fig.  31).  They  are 
all  formed  in  the  same  way,  by  the  formation  of  meri- 
stematic  layers,  which 
produce  cork  in  their  out- 
side and  add  to  the  cortex 
on  their  inner  faces,  be- 
having in  much  the  same 
way  as  the  cambium, 
though  the  cells  they 
form  do  not  give  rise 
to  bast  and  wood.  The  n 

,  ,  FIG.  31.  Section  of  a  lenticel,  /;  per, 

cork  is  impermeable  to  cork  layer. 

water  and  consequently 

all  the  cells  outside  the  innermost  layer  of  it  die ;  the 

tissue  thus  formed  constitutes  the  bark  (Fig.  32).     As  the 

years  go  on  it  becomes  thicker  and  thicker  and  much 


FIG.  30.  Commencement  of  cork 
formation  in  stem. 


THE  STRUCTURE  OF  THE  SHOOT 


75 


crinkled  and  split  up  through  the  action  of  the  weather 
and  the  storms  the  tree  is  exposed  to. 

We  must  now  examine  the  interior  arrangements  of 
the  leaf.  We  have  already  learnt  that  its  special  work 
is  mainly  twofold.  It  is  the  chief  agent  in  transpira- 


FIG.    33.    Section    of   bark  of   Quercus.      pe,    cork   layers    arising   at 
different  depths  in  the  cortex.       (After  Kny.) 

tion  or  the  evaporation  of  the  water  which  the  plant 
does  not  permanently  retain;  while  it  also  is  the  chief 
seat  of  the  construction  of  organic  food  substance. 
These  two  duties  are  discharged  mainly  by  the  cells  of 
the  lower  and  of  the  upper  halves  of  the  leaf  respectively. 
The  petiole  or  stalk  of  the  leaf  has  a  general  structure 
not  unlike  that  of  the  stem,  except  that  its  vascular 
strands  do  not  form  a  complete  cylinder,  but  only  half 


76 


BOTANY 


a  one,  being  open  on  the  upper  side  (Fig.  33).  The 
petiole  is  generally  not  cylindrical  in  shape,  but  flattened 
on  the  upper  face.  It  is  continued  with  little  change  of 
structure  into  the  blade,  where  it  appears  as  the  mid-rib. 
The  blade  of  a  stalked  leaf  is  the  ultimate  portion  of 
the  outgrowth  with  the  flattened  wing  which  has  been 

"  ep 


-co 


FIG.  33.  Section  of  petiole  of  Primula  sinensis.    ep,  epidermis;  co, 
cortex;   en,  endodermis ;   pe,  pericycle;   ph,  bast;   x,  wood. 

developed  along  its  two  edges.  The  part  between  the 
two  wings  is  generally  known  as  the  mid-rib.  If  we  cut 
a  section  through  the  blade  (Fig.  20)  we  find  its  structure 
adapted  to  the  work  for  which  the  flat  part  was  de- 
veloped. On  both  surfaces  we  find  an  epidermis,  the 
lower  one  especially  pierced  with  stomata.  Under  the 
upper  epidermis  the  cells  are  long  and  narrow  and 
arranged  side  by  side  much  like  the  vertical  railings 
of  a  fence.  They  touch  each  other  along  nearly  their 


THE  MONOCOTYLEDONOUS  PLANT        77 

whole  length,  intercellular  spaces  being  small  and  not 
numerous.  There  may  be  only  one  layer  or  several 
layers  of  these  cells,  which  constitute  what  is  known  as 
the  palissade  tissue.  The  cells  contain  numerous  chloro- 
plasts,  which  are  embedded  in  their  protoplasm.  Each 
has  as  usual  a  central  vacuole  filled  with  water.  These 
chloroplasts  are  capable  of  a  little  movement  in  the  celL 
It  is  in  this  layer,  exposed  to  the  light  most  freely,  that 
the  sugar  is  constructed.  (See  Fig.  20,  p.  52.) 

The  lower  half  of  the  leaf  is  made  of  cells  which  are 
spherical,  cubical,  or  oblong,  and  are  arranged  so  as  to- 
touch  each  other  only  at  few  points ;  consequently  the 
intercellular  passages  between  them  are  very  large, 
taking  up  sometimes  more  space  than  the  actual  cells. 
This  is  often  called  the  spongy  tissue  of  the  leaf.  The 
cells  contain  some  chloroplasts,  but  not  nearly  so  many 
as  the  palissade  cells.  This  layer  is  the  layer  in  which 
evaporation  occurs.  The  veins  generally  run  in  the 
centre  of  the  blade,  between  these  two  layers  of  cells. 

All  the  structures  of  both  petiole  and  blade  thus  show- 
exact  adaptation  to  the  two  main  duties  of  the  leaf. 


CHAPTER  VIII 

THE   MONOCOTYLEDONOUS   PLANT 

OUR  attention  has  been  mainly  directed  so  far  to  the 
peculiarities  of  the  dicotyledonous  plant.  We  must 
now  turn  for  a  little  while  to  study  another  form,  in 
which  the  embryo  has  only  one  cotyledon.  The  plants 
of  this  type  are  not  so  numerous  as  the  former  class,  but 
they  are  still  very  widespread.  The  most  easily  acces- 
sible of  them  in  this  country  are  the  grasses  and  the 
group  which  is  represented  by  the  common  white  lily. 

If  we  take  a  grain  of  wheat  we  have  what  is  very 
generally  spoken  of  as  the  seed  of  the  plant.     This  is 


78  BOTANY 

not  strictly  accurate ;  it  is  really  the  fruit  and  contains 
the  seed,  but  the  testa  of  the  seed  and  the  wall  of  the  fruit 
are  so  closely  united  that  we  cannot  separate  them.  The 
grain  of  wheat  is  a  small  ovoid  body  with  one  side  flattened 
and  grooved  down  its  length.  At  the  back, 
quite  at  the  lower  end  of  the  grain,  is  a  little 
.  wrinkled  area,  which  marks  the  position  of  the 
embryo,  above  which  and  forming  the  greater 
part  of  the  grain  is  the  endosperm,  filled  with 
food  for  the  young  plant  during  its  early  growth 
or  germination.  A  section  of  the  grain  is  shown 
in  Fig.  34. 

The  grass  embryo  possesses  a  single  large 
cotyledon  which  is  at  first  terminal  and  con- 
tinuous  in   a  straight  line  with  the  radicle, 
while  the  plumule  grows  out  laterally  some 
plG  little  distance  below  the  apex.      As  it  grows 

Longitudi-  the  cotyledon  becomes  forced  over  to  one  side, 
n^  s®?"0^  and  the  plumule  and  radicle  come  to  lie  in  a 
oatgrai    ^   straight  line,   as  in   the   dicotyledons.      The 
cotyledon  then  develops  along  the  side  of  the 
rest  of  the  embryo,  separating  it  from  the  endosperm. 
The  side  of  it  which  is  in  contact  with  the  latter  is  the 
part  which  absorbs  the  food  in  the  endosperm  cells.     In 
other  seeds  the  cotyledon  remains  in  a  line  with  the  radicle, 
the  whole  embryo  being  surrounded  by  the  endosperm. 
In  germination,  however,  the  upper  end  of  the  cotyledon 
is  the  last  to  leave  the  seed  coat,  remaining  there  and 
absorbing  the  endosperm  as  long  as  any  persists. 

If  we  soak  some  grains  of  wheat  or  barley  and  keep 
them  warm  germination  soon  begins.  The  radicle  pro- 
trudes as  a  little  white  body  from  the  micropyle ;  looking 
along  the  back  of  the  grain  we  can  notice  a  little  pointed 
prominence  gradually  making  its  way  in  the  other 
direction  under  the  skin  and  ultimately  emerging  at  the 
other  end ;  this  is  the  plumule.  As  it  gets  larger  it  dis- 


THE  MONOCOTYLEDONOUS  PLANT         79 


places  the  rest  of  the  grain,  which  comes  to  lie  on  the  side 
of  the  young  plantlet.     The  grain  remains  underground. 

The  further  development  is  differ- 
ent from  that  of  the  dicotyledon.  The 
root  does  not  form  a  tap  root,  but 
branches  almost  at  once;  indeed  in 
the  grasses  generally  it  begins  to  do 
so  before  it  escapes  from  the  grain. 
The  main  root  grows  scarcely  at  all, 
but  a  number  of  branches  arise  behind 
its  apex,  making  in  the  grasses  a 
cluster  of  delicate  fibrous  rootlets. 
The  growth  of  the  young  stem  is  seen 
more  advantageously  in  a  larger  grass 
—the  maize.  At  first  it  is  very 
slender,  but  as  development  proceeds 
its  growing  point  becomes  continu- 
ally larger  and  more  vigorous,  so  that 
each  node  and  internode  become 
larger  than  the  preceding  ones.  The 
young  stem  has  thus  the  form  of  an 
inverted  cone  (Fig.  35).  This  goes 
on  till  the  plant  reaches  a  certain 
height,  when  this  continuous  enlarge- 
ment ceases  and  the  later  parts  of  the 
stem  are  cylindrical.  Several  roots 
are  developed  from  the  nodes  of  the 
lower  part  of  the  stem  in  the  case 
of  most  monocotyledons ;  as  they 
arise  out  of  the  normal  order  they  are  called  adventitious 
roots. 

The  stem  of  the  monocotyledon  produces  as  a  rule 
one  leaf  at  each  node.  This  leaf  has  a  very  broad  base, 
which  encircles  the  stem  in  large  part,  or  sometimes 
entirely.  The  leaves  are  said  to  be  sheathing. 

The  general  requirements  of  the  plant  are  not  very 
different  from  those  of  the  dicotyledon  and  need  not 


G.  35-  Diagram  of 
mode  of  growth  of 
stem  of  monocoty- 
ledon (Zea).  (After 
Sachs.) 


8o 


BOTANY 


therefore  be  discussed  at  any  length.  The  distribution 
of  the  conducting  tissues  is,  however,  materially  different 
so  far  as  the  stem  is  concerned.  The  root  has  its  strands 
placed  like  those  of  the  dicotyledon,  but  it  never  increases 
in  thickness  and  does  not  show,  therefore,  any  develop- 
ment of  cambium  or  secondary  woody  elements.  The 
strands  in  the  stem  are  confined  to  the  central  cylinder, 
but  each  conjoint  strand  of  bast  and  wood  passes  up  the 

stem  separately.  Each  is 
surrounded  by  a  protecting 
sheath  of  hardened  cells 
and  never  contains  any 
cambium.  The  strands 
are  numerous  and  are 
arranged  in  a  series  of 
circles.  When  the  number 
is  very  great  this  circular 
arrangement  cannot  easily 
be  seen  and  the  strands 
appear  to  be  scattered 
thickly  through  the  central 
cylinder  (Fig.  36).  They 
are  found  to  be  continuous 
with  similar  strands  in  the  leaf,  which  as  before  are  called 
the  veins.  These  run  in  the  main  parallel  to  one  another, 
and  do  not  form  the  complex  network  which  is  seen  in 
the  leaf  of  a  dicotyledon. 

The  relations  of  the  leaf  to  the  stem  are  a  little 
different  from  those  of  most  dicotyledons.  The  bases  of 
the  leaves  sheathe  the  stem  and  do  not  as  a  rule  fall  off 
in  autumn.  The  leaf,  however,  sooner  or  later  dies,  but 
its  base  remains  where  it  was.  As  the  stem  grows 
older,  as  in  the  case  of  many  palms,  these  leaf  bases 
cover  nearly  the  whole  of  the  trunk,  causing  the  latter 
to  appear  much  thicker  than  it  really  is. 

The  leaf  has  palissade  parenchyma  in  a  narrow  layer 
under  both  surfaces. 


FIG.  36.  Diagram  of  transverse  sec- 
tion of  stem  of  monocotyledon. 


THE  FOOD  OF  PLANTS  81 

CHAPTER  IX 

THE   FOOD   OF   PLANTS 

WE  have  in  our  introductory  chapter  considered  in  out- 
line the  important  question  of  the  nutrition  of  plants,  a 
subject  which  has  been  treated  of  also  in  Chapter  VI.  of 
the  primer  of  Biology.  It  is  necessary  now  to  return  to 
this  subject  and  examine  it  a  little  more  fully. 

The  substances  that  are  taken  in  by  a  plant  are  the 
carbon  dioxide  which  is  present  in  the  air,  and  the  water 
and  dissolved  mineral  matters  which  the  roots  obtain 
from  the  soil.  We  must  again  emphasise  the  fact  that 
these  materials  are  not  capable  of  serving  as  food  in  the 
condition  in  which  they  are  absorbed,  but  that  a  great 
deal  of  work  has  to  be  carried  out  to  convert  them  into 
nutritive  material.  It  is  only  the  green  plant  which  can 
build  them  up  into  such  compounds  as  the  living  substance 
can  incorporate  into  itself,  the  work  being  effected  by 
the  chloroplasts,  the  little  ovoid  bodies  which  are  the 
seat  of  the  green  colouring  matter.  Further,  the  chloro- 
plasts can  only  work  when  they  are  properly  illuminated. 

The  carbon  dioxide  exists  in  very  small  proportion  in 
the  air,  not  more  than  about  3  parts  being  present  in 
10,000.  The  gas  enters  the  plant  by  way  of  the  stomata, 
and  so  makes  its  way  into  the  intercellular  spaces 
whence  it  obtains  access  to  the  cells  in  which  the  chloro- 
plasts are  present.  The  water  from  the  soil  is  conducted 
in  the  way  we  have  described  to  the  same  cells,  continu- 
ally replenishing  the  supply  in  their  vacuoles.  We 
have  thus  present  in  the  cells  of  the  parenchyma  of  the 
leaf  a  supply  of  carbon  dioxide,  water,  and  the  chloro- 
plasts themselves.  When  sunlight  shines  upon  the 
leaves  in  appropriate  intensity  the  constructive  action 
commences.  The  stages  are  not  yet  fully  understood, 
but  there  appears  to  be  no  doubt  that  in  some  way  the 
chloroplasts  cause  a  certain  chemical  action  to  be  set 

F 


82  BOTANY 

up ;  the  carbon  dioxide  and  some  of  the  water  disappear 
and  are  replaced  by  a  simple  organic  substance  known 
as  formaldehyde,  while  a  quantity  of  oxygen,  equal  in 
volume  to  the  carbon  -dioxide,  is  set  free.  The  oxygen 
finds  its  way  from  the  cells  into  the  intercellular  spaces 
and  passes  out  of  the  plant  by  way  of  the  stomata. 
Formaldehyde  is  thus  the  first  organic  product  which  is 
formed;  it  is  a  gaseous  body  and  probably  is  never 
present  in  any  but  very  small  quantities,  for  it  is  almost 
immediately  transformed  into  a  kind  of  sugar.  The 
manufacture  of  sugar  is  thus  the  first  stage  in  the 
preparation  of  the  food  of  the  plant. 

This  construction  of  sugar  cannot  be  carried  out 
without  the  application  of  energy.  We  are  familiar, 
from  our  ordinary  experience  of  things,  with  the  fact 
that  a  machine  cannot  be  made  to  do  work  without  a 
supply  of  energy.  A  steam  engine  cannot  work  without 
the  expenditure  of  a  certain  amount  of  fuel.  Whence 
then  does  the  chloroplast  obtain  the  energy  which  it 
applies  to  sugar-making?  The  answer  to  this  question 
explains  the  necessity  for  the  proper  illumination  which 
we  have  spoken  of  as  a  condition  of  its  activity.  The 
rays  from  the  sun,  which  we  speak  of  as  the  rays  of 
light,  are  absorbed  by  the  green  colouring  matter  of  the 
chloroplast.  We  can  prove  this  by  the  use  of  an  instru- 
ment known  as  the  spectroscope,  which  is  an  arrange- 
ment of  glass  prisms.  If  we  let  a  beam  of  white  light 
from  the  sun  fall  upon  such  a  prism  the  rays  of  which  it 
is  composed  are  bent  or  deflected  unequally  on  entering 
and  leaving  the  glass,  so  that  if  they  are  allowed  on 
emerging  to  fall  upon  a  plane  surface  they  appear  as  a 
broad  band  of  light  showing  a  series  of  colours  ranging 
in  order  from  red  to  orange,  yellow,  green,  blue,  and 
violet.  If  now  we  place  a  thin  film  of  a  solution  of 
chlorophyll  between  the  source  of  light  and  the  prism 
we  find  all  the  rays  do  not  reach  the  glass,  so  that  the 


THE  FOOD  OF  PLANTS  83 

coloured  band  which  emerges  is  not  continuous,  certain 
parts  of  it  being  blotted  out.  The  spectrum,  as  the 
band  is  called,  is  consequently  crossed  vertically  by  a 
number  of  dark  bands,  corresponding  to  the  position  of 
the  missing  rays.  In  the  living  cell  these  rays  are 
absorbed  by  the  chlorophyll  exactly  as  they  are  by  the 
solution  used  in  the  experiment,  and  it  is  from  them  the 
plant  derives  the  energy  which  is  used.  The  rays  which 
are  most  active  are  a  certain  number  of  the  red  ones; 
these  correspond  in  position  with  a  broad  black  band 
which  is  one  of  those  described. 

This  process  of  sugar  construction  can  only  take 
place  at  a  moderate  temperature. 

Another  very  important  constituent  of  the  plant's 
food  is  protein,  which  differs  from  the  group  of  food- 
stuffs to  which  sugar  belongs  by  containing  nitrogen  in 
combination.  Very  little  is  at  present  known  of  the 
processes  by  which  protein  is  made.  Compounds  of 
nitrogen,  preferably  nitrates  of  potassium,  calcium, 
magnesium,  or  ammonia,  are  absorbed  by  the  roots,  dis- 
solved in  the  water  which  they  take  in.  The  changes 
they  undergo  lead  in  some  still  unexplained  manner  to 
the  formation  of  much  more  complex  nitrogen  com- 
pounds, which  are  generally  though  not  strictly  accur- 
ately described  as  amides.  Among  them  may  be 
mentioned  asparagin,  leucin,  and  tyrosin. 

By  still  further  changes  these  are  converted  into 
proteins,  but  the  chemistry  of  the  process  is  still  obscure. 

A  third  constituent  of  the  food  of  plants  is  fat  or  oil. 
This  is  less  widely  distributed  and  appears  for  the  most 
part  only  in  places  of  storage.  It  is  formed  directly 
from  the  living  substance  there  and  does  not  appear  to 
be  built  up  from  simple  substances  in  the  plant. 

Sugar  is  a  member  of  a  group  of  substances  which  are 
called  carbohydrates.  It  is  formed  in  large  quantities 
when  the  chloroplasts  are  properly  illuminated;  much 


84  BOTANY 

more  is  made  than  is  needed  for  immediate  use.  The 
surplus  is  immediately  deposited  in  the  chloroplasts  in 
the  form  of  grains  of  starch.  Starch  is  another  carbo- 
hydrate which  very  readily  becomes  transformed  into 
sugar  by  the  action  of  dilute  mineral  acids  or  of  a 
peculiar  body  known  as  diastase,  which  is  a  digestive 
ferment  or  enzyme.  Probably  an  excess  of  protein  is 
also  made  while  the  conditions  are  favourable. 

The  cells  in  which  these  foodstuffs  are  made  have  to 
manufacture  sufficient  food  not  for  themselves  only,  but 
for  the  whole  plant,  many  of  whose  cells,  as  we  have 
seen,  are  set  apart  for  other  purposes  altogether.  It  is 
necessary  accordingly  for  the  food  to  be  made  in  large 
quantities  and  for  the  surplus  to  be  transported  from 
the  cells  in  which  it  is  made  to  the  other  cells  of  the 
organism.  Part  of  it  is  devoted  to  the  nutrition  and 
growth  of  the  body  of  the  plant,  and  a  large  amount  is 
stored  in  various  places  to  nourish  the  reproductive  organs. 

There  is  consequently  a  continuous  movement  of  the 
manufactured  food  substances  about  the  plant.  The 
sugar  temporarily  deposited  as  starch  grains  in  the 
chloroplasts  is  taken  away  as  soon  as  darkness  falls. 
The  ferment  or  enzyme  diastase,  which  is  present  in  the 
cells,  converts  the  starch  into  another  form  of  sugar, 
which  diffuses  outwards.  A  stream  of  such  sugar  is  thus 
leaving  the  leaf  during  the  night  and  passing  to  other 
cells  of  the  plant,  either  to  be  consumed  in  the  growth 
processes  or  to  be  stored  up  till  wanted.  Probably  this 
stream  of  sugar  is  passing  also  during  the  day,  but  while 
the  manufacture  is  going  on  it  is  not  easy  to  detect  it. 

When  proteins  leave  the  cell  they  are  first  converted 
by  a  similar  ferment  into  the  amides  we  have  spoken  of 
and  pass  through  the  plant's  tissues  as  such. 

Sugar  and  either  proteins  or  amides  are  taken  up  by  the 
living  protoplasm  and  incorporated  into  its  substance. 
This  is  the  true  assimilation,  or  nutrition  of  the  plant. 


THE  RESPIRATION 'OF  PLANTS  85 

The  foodstuffs  that  are  stored  instead  of  being 
directly  consumed,  undergo  a  transformation  which  is 
the  opposite  of  that  to  which  the  enzymes  give  rise. 
The  sugar  is  converted  again  into  starch,  a  process 
carried  out  by  certain  plastids  much  like  chloroplasts, 
but  without  any  green  pigment.  These  leucoplasts,  as 
they  are  called,  are  present  in  the  cells  in  which  the 
storage  takes  place.  The  amides  are  built  up  again  into 
proteins  and  deposited  in  the  cells.  In  seeds  they  appear 
as  peculiar  grains  of  protein  matter,  which  have  long  been 
known  under  the  name  of  aleurone  grains.  They  are  formed 
by  the  protoplasm  of  the  cell  and  not  by  any  plastid. 

The  transport  of  these  streams  of  food  material  is 
effected  chiefly  by  those  soft  parts  of  the  vascular 
strands  of  which  we  have  spoken  as  bast  or  phloem. 


CHAPTER  X 

THE   RESPIRATION   OF   PLANTS 

IT  is  a  matter  of  common  experience  with  us  all  that  a 
certain  process  called  breathing  must  take  place.  We 
know  that  we  are  continually  taking  air  into  our  bodies 
and  passing  it  out  again.  What  we  are  not  perhaps 
aware  of  is  that  the  air  we  give  out  differs  in  two  im- 
portant particulars  from  that  which  we  take  in — it  has 
gained  some  carbon  dioxide  and  it  has  lost  some  oxygen. 
What  is  true  of  ourselves  is  true  also  of  plants. 
During  the  whole  time  of  their  lives  they  are  absorbing 
oxygen  and  giving  out  carbon  dioxide,  two  processes 
which  constitute  the  beginning  and  the  end  of  another 
very  complex  internal  one  which  is  known  as  respiration. 
So  long  as  active  life  lasts  this  interchange  of  gases  can 
be  detected  by  appropriate  methods,  though  it  is 
observed  with  difficulty  during  the  daytime  on  account 
of  the  absorption  of  carbon  dioxide  and  liberation  of 


86  BOTANY 

oxygen,  which  we  have  seen  to  be  associated  with  the 
manufacture  of  sugar.  It  can,  however,  be  detected  by 
experiments.  To  prove  the  absorption  of  oxygen,  take 
a  flask  and  fit  it  with  an  india-rubber  stopper  through 
which  a  glass  tube  bent  twice  at  right  angles  can  be 
passed;  let  the  end  of  the  tube  dip  into  mercury  in  a 
small  vessel  standing  side  by  side  with  the  flask.  Fill 
the  flask  with  healthy  leaves.  In  addition  to  the  leaves 
place  in  the  flask  a  test  tube  containing  a  solution  of 
caustic  potash  and  cork  it  up  so  that  the  outlet  tube 
dips  into  the  mercury.  Keep  it  at  a  constant  tempera- 
ture for  some  hours.  The  mercury  will  rise  gradually 
into  the  tube,  and  will  continue  to  do  so  for  some  time, 
so  showing  that  a  diminution  of  the  volume  of  the  air  in 
the  flask  has  taken  place.  Analysis  of  the  air  then  left 
in  the  flask  will  show  that  the  volume  of  nitrogen  is  un- 
changed, while  that  of  oxygen  has  diminished.  Exami- 
nation of  the  caustic  potash  will  show  that  it  has 
absorbed  an  amount  of  carbon  dioxide. 

The  reason  for  employing  the  potash  is  that  the 
carbon  dioxide  which  is  exhaled  in  respiration  is  about 
equal  in  volume  to  the  oxygen  absorbed,  so  that  unless 
it  is  removed  the  diminution  of  the  volume  of  the 
oxygen  will  be  compensated  for  by  the  addition  of  the 
carbon  dioxide  and  the  mercury  will  not  rise  in  the  tube. 

We  can  show  the  exhalation  of  carbon  dioxide  by  the 
living  plant  by  means  of  another  experiment.  Place  a 
number  of  leaves  in  a  flask  through  the  stopper  of  which 
air  can  be  made  to  enter  by  one  glass  tube  and  can  be 
removed  by  another.  Make  the  air  as  it  enters  pass 
through  a  bottle  containing  a  solution  of  caustic  potash 
which  will  free  it  from  all  traces  of  carbon  dioxide. 
Pump  the  air  through  the  flask  by  means  of  some  form 
of  air  pump  or  aspirator  and  lead  it  through  a  bottle  of 
lime-water.  It  will  soon  make  the  lime-water  milky  in 
consequence  of  the  formation  of  carbonate  of  calcium, 


THE  EVOLUTION  OF  PLANTS  87 

brought  about  by  the  reaction  of  carbon  dioxide  with 
the  lime.  As  all  carbon  dioxide  was  kept  from  entering 
the  flask,  and  as  the  lime-water  shows  by  becoming 
milky  that  some  of  this  gas  reached  it  in  the  stream  of 
air  pumped  through  it,  it  becomes  perfectly  clear  that 
the  plant  has  exhaled  it.1 

Besides  carbon  dioxide^  certain  amount  of  water  vapour 
is  exhaled  by  the  plant  in  the  process  of  respiration. 

To  understand  the  purpose  of  respiration  we  must  go 
back  a  little  and  consider  some  of  the  features  of  sugar 
manufacture.  We  saw  that  the  energy  for  this  work  is 
derived  from  the  sun.  The  work  it  does  is  to  build  up 
sugar  and  subsequently  other  compounds,  which  remain 
in  the  plant.  These  substances  retain  in  themselves  the 
energy  which  was  expended  in  making  them.  When 
they  are  decomposed  or  reduced  to  simple  compounds 
like  those  from  which  they  were  made,  this  energy  can 
be  liberated  again.  If  we  burn  them,  for  instance,  we 
get  the  liberation  of  a  large  amount  of  heat,  which  can 
be  made  to  do  work  in  other  directions.  So  we  see 
that  the  construction  of  the  plant,  both  living  and  non- 
living substance,  involves  the  fixation  of  large  amounts 
of  energy  as  well  as  of  material. 

The  purpose  of  respiration,  which  involves  the  break- 
ing down  of  the  living  substance  incidental  to  its  wear 
and  tear  and  the  work  it  does,  is  in  this  way  to  liberate 
the  energy  without  which  these  operations  could  not 
take  place. 

CHAPTER  XI 

THE  EVOLUTION  OF  THE  FORMS  OF  PLANTS— ALG^ 

THE  considerations  set  out  in  the  foregoing  chapters 
apply  in  great  part  only  to  the  higher  terrestrial  plants. 
But  the  whole  range  of  vegetation  is  much  more  exten- 

1  A  form  of  the  apparatus  is  shown  in  the  primer  of  Biology,  p.  56. 


88  BOTANY 

sive  than  this.  There  are  many  other  types  of  plants 
which  differ  from  the  land  plant  we  have  considered 
and  differ  too  among  themselves  in  many  respects. 
They  live  in  various  situations,  they  attain  to  very 
different  dimensions,  and  they  show  a  great  variety  in 
the  details  of  their  internal  structure.  A  very  large 
number  of  plants,  some  very  humble,  others  very 
elaborate,  in  degree  of  development,  live  altogether  or 
almost  entirely  in  water.  Many  others  of  simple  struc- 
ture occupy  moist  situations,  such  as  rocks  on  the  banks 
of  streams,  damp  earth,  or  trunks  of  trees  near  the 
ground;  others  again  dwell  in  hot,  arid  regions  where 
little  water  can  reach  them.  Nearly  all  these  forms 
resemble  the  higher  plants  in  possessing  the  green 
colouring  matter;  but  there  are  others  which  have  lost 
it,  and  which  live,  therefore,  on  decaying  matter  or  in 
the  bodies  of  other  plants  or  of  animals. 

If  we  consider  the  whole  mass  of  vegetation  and  com- 
pare the  simple  forms  we  find  with  others  of  much 
complexity,  we  come  to  see  that  there  is  and  has  been 
throughout  it  a  continual  advance,  though  a  very  slow 
one,  in  the  direction  of  greater  complexity.  This  leads 
us  to  believe  that  the  most  highly  organised  plants  we 
see  to-day  have  been  developed  from  extremely  simple 
ones  during  the  long  ages  of  the  past. 

If  we  try  to  determine  how  this  has  taken  place  we 
are  able  to  form  some  idea  of  its  course  by  studying  the 
simple  plants  existing  at  the  present  time  and  the 
gradual  increase  in  complexity  we  can  find  among  them. 
Very  probably  the  different  forms  show  us  the  different 
stages  through  which  development  has  passed. 

The  simplest  plant  we  find  to-day  is  a  very  small 
structure  living  in  water.  It  consists  of  a  single  piece 
of  protoplasm  with  its  nucleus ;  it  is  clothed  by  a  thin 
cell  wall  and  contains  the  green  colouring  matter.  We 
can  suppose  without  much  fear  of  mistake  that  the  first 


THE  EVOLUTION  OF  PLANTS  89 

plant  that  existed  was  not  very  different  from  this,  and 
like  it  dwelt  in  water.  Nor  are  we  likely  to  be  wrong 
in  thinking  that  the  whole  group  of  the  seaweeds  arose 
by  gradual  development  from  such  a  form,  perhaps 
before  any  plants  grew  upon  the  surface  of  the  land. 
If  we  examine  this  group  of  seaweeds  we  find  a 
succession  of  forms  which  are  step  by  step  more 
elaborate,  and  we  see  therein  some  justification  for 
thinking  that  development  or  evolution  has  proceeded 
on  similar  lines.  There  are  two  points  that  may  well 
be  insisted  upon  as  a  preliminary  to  thinking  about 
these  changes:  first,  there  is  a  predisposition  in  the 
living  substance  to  become  more  highly  organised; 
second,  this  organisation  is  brought  about  by  the  action, 
direct  or  indirect,  of  the  surroundings,  the  plant  suc- 
ceeding best  which  is  in  the  most  complete  harmony 
with  them.  We  have  already  illustrated  the  second  of 
these  points  so  far  as  the  higher  land  plant  is  concerned. 

It  is  possible  that  the  original  ancestors  of  the  sea- 
weeds were  even  simpler  than  the  one  we  have  described, 
not  possessing  any  cell  wall.  If  so,  the  exposure  of  the 
living  substance  to  the  changes  in  the  water  and  to 
contact  with  obstacles  in  it  no  doubt  led  to  the  formation 
of  this  cell  wall  for  purposes  of  protection. 

The  development  of  this  membrane  round  it,  how- 
ever, introduced  a  difficulty  in  its  relations  with  the 
water,  free  access  to  which  for  so  many  reasons  was  very 
necessary  to  its  welfare.  We  can  understand  that  the 
development  of  the  vacuole,  with  its  little  store  of  water, 
was  speedily  effected  to  obviate  this  difficulty. 

Increase  of  size  of  the  protoplast  was  no  doubt  one  of 
the.  early  features  of  the  life  of  the  plant.  This  in  the 
condition  of  a  cell  clothed  by  a  membrane  was  only 
possible  up  to  a  certain  point  without  weakening  the 
membrane  and  so  disturbing  the  protection  it  had 
secured.  Growth  was  consequently  followed  by  divi- 


90  BOTANY 

sion  into  two  and  the  gradual  separation  of  the  two  from 
one  another.  Here  we  see  the  simplest  form  of  repro- 
duction following  growth,  each  of  the  two  protoplasts 
resulting  from  the  division  possessing  the  same  proper- 
ties as  the  original  cell.  When  in  some  cases  the 
processes  of  growth  and  division  became  very  rapid,  a 
second  division  might  well  take  place  in  the  two  new 
cells  before  they  had  become  separated.  There  would 
arise  in  this  way  by  degrees  a  new  form  of  plant,  one  in 
which  the  processes  of  division  continued,  but  the  cells 
did  not  separate  at  all,  so  that  a  chain  or  filament  of 
cells  resulted.  They  would  keep  their  individuality  at 
first  at  any  rate,  being  almost  as  independent  of  each 
other  as  if  they  had  separated,  their  attachment  being 
mainly  mechanical.  We  find  a  very  large  number  of 
water-weeds,  or  algae,  existing  in  our  ponds  and  streams 
to-day  which  have  just  this  structure. 

The  maintenance  of  this  thread-like  structure  depends 
on  the  cells  always  dividing  in  such  a  way  that  the  new 
cell  walls  arise  across  the  length  of  the  thread.  It  seems 
certain  that  this  course  was  soon  departed  from  by  some 
of  the  plants  and  that  some  divisions  arose  at  right 
angles  to  the  others.  This  led  to  the  formation  of  a  flat 
plate  of  cells,  still  only  one  cell  thick.  A  plant  with  this 
structure  would  meet  with  very  little  greater  difficulty 
than  the  thread-like  forms ;  the  requirements  of  the 
cells  would  be  the  same,  though  the  plant  might  easily 
grow  very  much  larger,  the  number  of  its  protoplasts  or 
cells  often  amounting  to  many  hundreds.  Still  the  cells 
would  be  all  alike  and  probably  all  independent,  for 
water  would  have  access  to  each  and  no  further  pro- 
vision would  be  called  for.  Plants  of  this  type  of 
structure  are  still  common  among  marine  seaweeds. 

A  great  complexity,  however,  would  arise  if  the 
protoplasts  began  to  divide  in  three  planes,  each  at 
right  angles  to  the  other  two.  No  doubt  this  was  not 


THE  EVOLUTION  OF  PLANTS  91 

long  delayed,  and  plants  began  to  possess  a  bulk  or 
mass,  instead  of  being  filaments  or  one-layered  plates. 
This  was  a  most  important  change,  for  it  altered  the 
relations  between  the  cells  or  protoplasts  of  the  plant 
and  the  surrounding  water.  Only  the  external  cells  in 
such  a  mass  were  able  to  absorb  water  and  the  inner  ones 
had  to  depend  on  them  for  a  supply.  The  external 
cells,  too,  were  those  which  ran  the  greatest  risks  from 
changes  of  temperature  in  the  water  and  from  contact 
with  particles  in  it,  or  the  many  dangers  which  the 
environment  brings  to  the  plant.  These  dangers  and 
difficulties  must  have  increased  as  the  plant  itself  grew 
larger.  The  difficulties  of  the  internal  cells  were 
different,  but  no  less  serious.  They  were  compelled  to 
draw  upon  the  external  ones  for  the  renewal  of  the 
water  in  their  vacuoles,  for  the  oxygen  necessary  for 
breathing,  for  their  food  or  its  constituents,  and  for  the 
removal  of  any  waste  products  they  might  produce.  So 
the  two  became  gradually  less  and  less  like  each  other,  or 
more  specialised.  The  gradual  change  of  structure  and 
form  of  plants  that  followed  can  be  traced  in  this  way  to 
the  need  for  adapting  them  to  their  conditions  of  life. 

We  can  follow  with  some  probability  the  further 
course  of  events.  As  the  bulk  increased  the  external 
cells  became  still  more  devoted  to  protection  and  absorp- 
tion ;  the  internal  ones  ceased  to  develop  chlorophyll  as 
less  light  reached  them,  and  the  work  of  food  construc- 
tion which  is  the  function  of  the  chlorophyll  was  thus 
thrown  upon  the  outer  layers.  Among  the  internal 
mass  certain  cells  became  concerned  mainly  in  conduct- 
ing the  water  to  the  rest,  so  supplying  them  with  oxygen 
and  food.  As  the  size  became  still  greater  the  exterior 
surface  of  the  still  symmetrical  spheroidal  or  spherical 
plant  became  inadequate  to  supply  all  that  the  inner 
mass  required;  the  spherical  plant  no  longer  had  suffi- 
cient surface  in  proportion  to  its  bulk.  This  involved 


92  BOTANY 

the  restriction  of  growth  mainly  to  certain  regions  of  the 
surface,  which  became  what  we  now  call  growing  points  ; 
here  the  multiplication  of  cells  led  to  the  formation  of 
conical  outgrowths,  and  these  in  turn  were  soon  recognis- 
able as  branches.  The  larger  the  plant  became  the 
more  necessary  was  it  for  it  to  become  branched  in  view 
of  the  dangers  from  storms,  and  currents  in  the  water, 
as  it  would  oppose  much  less  resistance  to  the  move- 
ments of  the  stream  or  tide. 

The  problem  of  the  passage  of  the  absorbed  water,  as 
it  made  its  way  from  exterior  to  interior  cells,  involved 
the  question  of  its  being  able  to  pass  through  the  cell 
walls  that  lay  in  its  way.  As  the  length  of  the  body  of 
the  plant  increased  with  the  putting  out  of  branches,  the 
number  of  the  cell  walls  became  an  inconvenient  obstacle 
to  its  passage.  The  flow  being  in  a  particular  direction 
caused  the  cells  to  stretch  a  little  accordingly  and  so  to 
make  them  become  a  little  longer  than  broad.  This 
elongation  soon  became  advantageous,  as  with  cells  of 
that  shape  there  were  fewer  walls  to  pass  in  a  given 
distance.  So  gradually  in  the  regions  of  transport 
elongated  cells  became  most  usual.  After  a  time  the 
end  walls  became  perforated  and  later  still  dissolved 
altogether,  so  that  the  water  could  pass  easily  along  a 
tract  which  was  originally  a  number  of  columns  of  cells. 
We  can  observe  this  change  of  internal  structure  taking 
place  in  many  plants  to-day.  Each  column  when  it 
has  lost  its  end  walls  forms  a  vessel.  We  find  them 
fully  developed  only  in  terrestrial  plants,  but  some  of 
the  larger  seaweeds  contain  structures  much  like  them, 
the  end  walls  being  perforated  very  freely. 

During  the  time  these  changes  were  taking  place 
in  the  arrangements  of  the  interior,  other  signs  of 
specialisation  of  exterior  parts  made  themselves  visible. 
With  increase  of  size  and  great  flexibility  of  body  as  in 
the  filamentous  forms  it  became  advantageous  to  be 


DEVELOPMENT  OF  SEAWEEDS  93 

attached  to  some  substratum  and  no  longer  passively 
floated  about.  So  attaching  organs  were  developed; 
in  some  of  the  simple  filaments  they  were  only  a  modi- 
fied cell  at  the  basal  end  of  the  thread ;  in  the  bulky 
plants  the  whole  of  the  unbranched  end  was  often 
specially  modified  to  form  a  kind  of  grasping  organ 
capable  of  growing  round  stones  or  into  crevices. 

We  find  such  anchorage  mechanisms  still  in  the  larger 
seaweeds  of  our  coasts.  These  organs  are  not  true 
roots,  for  they  anchor  the  plant  only,  and  do  not  absorb 
nutritive  or  mineral  compounds  for  its  use. 

Such  a  development  of  the  body  of  the  plant  was 
sufficient  for  aquatic  plants  such  as  seaweeds.  Even  in 
the  largest  of  them  there  is  only  a  very  slight  specialisa- 
tion of  structure  compared  with  that  which  is  necessary 
for  plants  when  they  come  to  live  on  land.  We  can, 
however,  find  in  many  present  day  forms  three  systems 
of  tissues,  an  external  protective  and  absorbing  coat,  a 
conducting  system,  and  between  them  a  system  of  cells 
belonging  to  neither,  whose  function  so  far  has  not  been 
very  apparent  to  us.  We  see  that  the  division  of  the 
body  of  the  plant  into  members  does  not  go  so  far  as  to 
enable  us  to  recognise  what  we  call  stem,  leaf,  or  root. 
It  is  in  some  cases  undivided,  in  others  very  much 
branched,  but  its  structure  is  practically  the  same 
throughout.  We  speak  of  such  a  form  of  plant  as  a 
thallus,  whether  branched  or  simple,  massive  or  minute. 


CHAPTER  XII 

THE    DEVELOPMENT   OF   THE    REPRODUCTIVE 
PROCESSES   IN   THE   ALG^E 

WHILE  these  changes  were  occurring  in  the  form  and 
structure  of  the  body  of  the  plant,  other  developments 
were  taking  place  in  the  ways  in  which  new  individuals 


94  BOTANY 

arose.  While  plants  had  only  a  unicellular  body,  the 
division  of  the  cell  caused  the  appearance  of  a  new  in- 
dividual. In  fact,  the  individual  was  the  same  thing  as 
the  cell.  But  when  the  cells  failed  to  separate  this 
ceased  to  be  the  case,  and  the  individual  plant  became 
identified  with  the  chain  of  cells.  Division  of  a  cell  so 
added  another  link  to  the  chain,  but  it  did  not  produce  a 
new  individual.  For  a  time  the  simple  method  of  re- 
production by  cell  division  was  replaced  by  the  break- 
ing of  the  chain  into  a  number  of  segments,  each  of 
which  grew  into  a  new  filament  like  the  first.  This 
method  of  reproduction  is  shown  now  by  certain  blue- 
green  algae  and  by  many  fungi. 

These  simple  methods,  however,  led  to  the  production 
of  comparatively  few  offspring.  A  larger  number 
became  desirable  if  a  species  was  to  hold  its  own  in  the 
competition  with  its  neighbours  for  what  the  surround- 
ings afforded,  particularly  as  the  life  of  any  individual 
was  but  short.  So  there  arose  in  the  plant  body  special 
cells,  usually  in  great  numbers,  which  could  be  detached 
and  give  rise  to  new  individuals.  Various  forms  of 
these  cells  are  still  met  with ;  some  with  no  membrane, 
able  to  swim  about  by  means  of  little  protoplasmic 
thread-like  outgrowths,  known  as  cilia,  which  by  rapid 
vibrations  set  up  currents  in  the  water ;  some  motion- 
less and  well  protected  by  firm  cell  walls,  so  as  to  be  able 
to  resist  heat  or  cold,  and  even  some  degree  of  drying 
up.  Such  little  specialised  reproductive  cells  are  now 
generally  called  gonidia.  They  are  still  produced  in 
various  ways  and  usually  in  very  large  numbers  by 
many  of  the  seaweeds  and  another  group  related  to 
them — the  fungi.  This  showed  a  great  advance  in  the 
spread  of  the  species,  as  each  of  the  large  number 
produced  could  give  rise  to  a  new  plant. 

But  such  rapid  reproduction  tends  to  weaken  the  race, 
and  no  doubt  it  told  its  tale  in  the  ancient  times.  It 


DEVELOPMENT  OF  SEAWEEDS  95 

was  supplemented  by  another  method  which  gave  rise 
to  a  renewal  of  vigour  in  the  offspring.  How  the  new 
method  was  first  brought  about  it  is  hard  to  say.  It 
can  still  be  seen  to  play  its  part  among  the  free  swim- 
ming gonidia  of  the  seaweeds.  Two  of  them,  instead  of 
developing  into  two  plants,  fuse  completely  together  to 
form  a  single  protoplast,  which  after  a  period  of  rest 
grows  into  a  single  new  plant.  This  union  of  the  two  is 
called  conjugation  and  the  fused  product  is  a  zygote. 

We  have  in  this  process  an  indication  of  the  way  in 
which  sex  in  plants  began  to  be  developed.  We  cannot 
speak  of  either  of  the  two  conjugating  cells  in  this  stage 
as  male  or  female,  but  the  further  development  into 
well-recognisable  sexes  can  be  easily  traced.  One  of  the 
cells  of  the  pair  became  larger  and  more  sluggish  than 
the  other ;  then  a  stage  was  reached  in  which  the  larger 
of  the  two  was  not  motile  at  all.  By  and  by,  instead  of 
being  developed  from  any  cell  of  the  plant  they  came 
to  be  produced  in  particular  cells  or  groups  of  cells — 
organs  for  their  development.  The  smaller  active  cells 
came  to  be  recognised  as  male,  the  sluggish  ones  as 
female.  At  first  they  were  equally  numerous,  but  as  the 
females  became  larger  fewer  of  them  were  formed  in  the 
specialised  cells.  The  number  varied  indeed  inversely 
as  the  size.  While  the  males  continued  to  be  produced 
in  large  numbers  the  females  became  ultimately  solitary 
in  the  organs  bearing  them.  Finally  the  female  never 
escaped  from  the  organ  but  was  joined  there  by  a  male. 
The  act  of  fusion  of  the  two  has  come  to  be  spoken  of  as 
fertilisation.  The  female  cell  is  now  known  as  an  ovum ; 
the  male  cells  are  called  sperms.  There  is  an  almost 
infinite  variety  in  the  modifications  which  these  lowly 
plants  have  shown  and  show  to-day  in  the  ways  they 
develop  their  gonidia  and  their  ova  and  sperms.  We 
cannot  go  more  deeply  into  the  matter  here. 

From  the  elaborate  nature  of  its   mechanism,   this 


96  BOTANY 

sexual  process  of  reproduction  led  to  the  appearance  of 
fewer  offspring  than  the  method  of  gonidia  production, 
but  those  so  formed  were  much  more  vigorous  and  better 
qualified  in  every  way  to  carry  out  their  vital  processes, 
and  hence  this  process  has  become  almost  universal. 

This  stage  does  not,  however,  mark  finality  among 
plants.  It  involved  a  good  deal  of  uncertainty  as  to 
whether  fertilisation  would  take  place,  as  both  the 
parents  of  the  sperm  cells  and  the  ova  were  unable  to 
move  except  as  they  were  drifted  about  in  the  water. 
The  sperm  retained  the  power  of  swimming  when 
liberated,  but  the  ovum  in  the  higher  types  remained 
enclosed  in  the  oogonium  or  cell  in  which  it  was  formed. 
This  uncertainty  led  to  a  further  development,  by  which 
one  act  of  fertilisation  was  made  to  produce  several 
young  plants  instead  of  one.  This  was  brought  about 
by  the  zygote — the  fertilised  female  cell — dividing  up 
into  a  number  of  cells,  which  were  liberated  by  the 
bursting  of  the  zygote  wall.  These  cells  in  most  cases 
were  furnished  with  cilia  and  in  general  appearance  and 
behaviour  were  hardly  if  at  all  distinguishable  from 
the  gonidia  described  before.  Such  reproductive  cells 
can  be  seen  to-day  in  the  alga  (Edogonium.  To  distin- 
guish them  from  the  gonidia  they  are  spoken  of  as 
spores.  One  act  of  fertilisation  results  in  this  way  in 
the  production  of  a  number  of  new  individuals. 

A  little  further  advance  still  was  made  among  the 
algae,  which  became,  however,  much  more  marked 
among  the  plants  which  established  themselves  on  land. 
Instead  of  the  fertilised  cell  dividing  at  once  to  produce 
the  spores,  it  developed  into  a  relatively  bulky  multi- 
cellular  structure,  which  became  differentiated  so  that 
only  part  of  it  gave  rise  to  the  spores,  while  the  rest 
served  to  protect  them  and  to  minister  to  their  nutrition. 
Such  a  structure  can  be  seen  to-day  in  many  of  the  red 
seaweeds,  in  which  it  is  known  as  a  sporocarp. 


EVOLUTION  OF  THE  LAND  PLANT   97 

In  the  land  plants,  as  we  shall  see  presently,  this  line 
of  development  became  much  more  extended.  The 
zygote  gave  rise  to  a  much  more  highly  organised  sporo- 
carp,  in  which  the  cells  which  formed  the  spores  became 
more  restricted  in  their  numbers  and  disposition.  The 
further  progress  of  development  led  to  much  greater 
differentiation  of  the  sporocarp,  which  gave  it  the  power 
of  living  independently.  The  sterile  part,  or  the  region 
which  did  not  directly  form  spores,  became  much  en- 
larged and  grew  to  dimensions  much  exceeding  those  of 
the  original  thallus.  Its  body  became  differentiated 
into  root,  stem,  and  leaves,  and  upon  the  sub-aerial 
parts  the  spores  were  formed  in  structures  known  as 
sporangia.  This  structure,  known  as  the  sporophyte,  is 
now  the  dominant  form  of  all  terrestrial  plants.  We 
shall  consider  this  change  in  the  next  chapter. 


CHAPTER  XIII 

THE   ORIGIN   OF   TERRESTRIAL   PLANTS — EVOLUTION 
OF  MOSSES   AND   FERNS 

WE  must  now  consider  what  has  been  perhaps  the  most 
important  step  we  find  in  the  development  of  vegetation, 
the  transference  of  plants  from  water  to  land.  At  first 
all  were  aquatic,  but  in  the  natural  course  of  events  no 
doubt  many  were  washed  on  to  the  moist  earth  by  the 
side  of  the  water  in  which  they  were  living.  Being 
adapted  only  to  life  in  water  no  doubt  most  perished, 
and  it  was  only  gradually  that  some  established  them- 
selves. A  comparison  of  aquatic  with  terrestrial  forms 
as  we  find  them  to-day  shows  how  complete  a  change 
in  almost  every  respect  took  place.  In  the  water  the 
direction  of  their  growth  was  comparatively  unimportant, 
and  a  large  number  grew  in  a  horizontal  position.  On 
land  we  find  this  extremely  rare;  most  plants,  as  we 

G 


98  BOTANY 

have  seen,  grow  vertically  upwards  and  downwards,  and 
show  very  different  peculiarities  in  the  parts  which  take 
these  opposite  directions.  The  transference  probably 
did  not  take  place  till  the  habits  of  reproduction  we 
have  briefly  described  had  been  acquired.  Most  likely 
the  first  form  which  secured  a  footing  in  the  soil  was  a 
flattened  thallus,  consisting  perhaps  of  only  a  few  cells. 
Its  small  size  would  enable  it  to  touch  the  moist  earth 
over  a  large  part  of  its  surface  and  so  to  be  able  to 
absorb  the  water  it  required.  But  with  multiplication 
of  cells,  and  consequent  increase  of  size,  this  became  dim- 
cult  or  impossible.  It  could  only  reach  the  moisture  in 
places  and  the  supply  so  became  insufficient.  We  find 
such  plants  now  on  the  earth,  and  we  see  that  to  secure 
the  water  supply  they  have  developed  outgrowths  of 
their  surface  cells  which  resemble  root  hairs  in  struc- 
ture; these — the  rhizoids — adhere  very  closely  to  the 
surface  and  gradually  penetrate  between  the  particles  of 
earth,  so  burying  themselves  in  the  soil.  They  become 
very  closely  attached  to  its  particles,  so  that  they  are 
able  to  absorb  the  film  of  water  by  which  each  particle 
is  surrounded.  They  are  produced  in  great  numbers 
and  are  continually  being  renewed.  It  is  probable  that 
it  was  by  such  an  arrangement  that  the  early  terrestrial 
plants  were  enabled  to  establish  themselves. 

The  difficulties  which  they  encountered  caused  them 
to  produce  as  many  and  as  vigorous  offspring  as  possible, 
and  as  in  consequence  of  the  scarcity  of  water  fertilisa- 
tion became  more  and  more  difficult,  the  sperms  not 
being  brought  into  the  neighbourhood  of  the  ova,  they 
gradually  came  to  develop  an  increasing  number  of 
spores  from  each  fertilised  cell.  This  was  advantageous 
in  another  way  as  the  spores  were  more  capable  of  resist- 
ing the  adverse  conditions  than  their  parent,  owing  to 
their  simpler  structure,  their  more  moderate  require- 
ments, and  their  thicker  outer  membrane.  On  the 


EVOLUTION  OF  THE  LAND  PLANT        99 

other  hand,  the  multiplication  of  the  young  plants  to 
which  the  spores  gave  rise  made  competition  much  more 
severe.  The  young  plants  were  produced  so  close  to 
each  other  as  to  cause  great  overcrowding,  making  it 
difficult  for  each  to  be  properly  illuminated  and  to 
secure  sufficient  nutritive  material  from  the  soil. 

The  flattened  form  of  plant  under  these  conditions 
became  unsuitable,  and  each  plant  was  compelled  to 
grow  upwards  or  perish.  The  rhizoids  in  this  way 
tended  to  accumulate  on  the  comparatively  small  part 
of  the  thallus  which  was  left  nearest  to  the  soil  so  that 
the  rest  might  raise  itself  into  the  air.  Gradually  thus 
the  plant  came  to  show  the  descending  portion  adapted 
to  live  buried  in  the  soil  and  the  ascending  portion 
freely  exposed  to  light  and  air.  We  find  thus  an  indica- 
tion of  the  parts  we  have  spoken  of  as  the  root  and  the 
shoot.  It  must  not  be  concluded,  however,  that  these 
early  plants  showed  either  in  such  a  state  of  development 
as  we  recognise  in  the  land  plants  of  to-day. 

The  methods  of  reproduction  gradually  underwent 
much  modification,  as  those  of  the  aquatic  plants  were 
but  slightly  suitable  for  erect  plants,  only  part  of  which 
could  have  free  relations  with  water  or  moist  earth. 
The  change  of  position  rendered  fertilisation  uncertain, 
and  it  became  more  and  more  difficult  for  the  sperms  to 
get  to  the  ova.  To  help  the  process  to  take  place,  more 
elaborate  structures  were  formed  in  which  to  develop 
the  sperms  and  the  ova.  These,  known  as  antheridia 
and  archegonia,  were  produced  in  the  neighbourhood  of 
moisture  as  far  as  possible,  often  on  the  under  side  of 
the  flattened  thallus,  or,  in  the  case  of  an  erect  form,  on 
the  parts  liable  to  be  moistened  by  rain  or  dew.  The 
result  of  the  difficulties  associated  with  fertilisation 
was  a  great  development  of  the  structure  to  which  the 
fertilised  ovum  gave  rise,  which  produced  large  numbers 
of  spores.  Gradually  this  became  more  and  more 


ioo  BOTANY 

elaborate,  and  instead  of  originating  nothing  beyond 
the.  spores  and  certain  outgrowths  to  protect  them,  it 
grew  to  be  self-supporting,  by  producing  cells  or  areas 
of  cells  which  contained  the  chlorophyll.  Before  it 
succeeded  in  making  chlorophyll  for  itself  it  was  com- 
pelled to  derive  all  its  nourishment  from  the  plant  which 
bore  the  ovum  that  gave  rise  to  it.  This  ovum  never 
left  the  archegonium  in  which  it  arose.  The  sporocarp, 
as  the  spore-containing  structure  is  called,  originated 
accordingly  in  the  cavity  of  the  archegonium  and  was 
able  thus  to  feed  on  the  parent  plant. 

We  find  this  stage  in  the  evolution  of  the  land  plant 
represented  to-day  by  the  mosses.  The  plants  which 
bear  the  ova  and  sperms  are  very  small,  seldom  more 
than  an  inch  in  height.  They  have  a  very  slender  stem, 
bearing  a  number  of  delicate  leaves,  and  are  anchored 
to  the  soil  by  a  number  of  rhizoids  which  spring  from 
the  bottom  of  the  stem  (Figs.  37,  40  B).  The  sperms 
are  produced  in  antheridia,  which  may  be  found  at  the 
tops  of  some  of  the  stems  among  the  crowd  of  leaves 
arising  there  (Fig.  38).  The  ova  are  similarly  situated 
at  the  tops  of  other  stems ;  each  is  developed  singly  in 
an  archegonium,  a  bottle-shaped  body  with  a  long  neck 
(Fig.  39).  The  sperm  can  only  reach  the  archegonium 
when  the  moss  plants,  which  grow  thickly  together,  are 
wetted  by  rain  or  dew,  as  it  must  transport  itself  by 
swimming.  When  it  reaches  the  archegonium  it  makes 
its  way  down  the  neck  of  the  latter  and  fuses  with  the 
ovum  in  the  swollen  basal  part.  The  stem  and  leaves 
of  the  moss  plant  are  very  simple  in  structure;  the 
former  shows  a  protective  outer  layer  or  epidermis  and 
an  interior  mass  of  delicate  thin-walled  cells.  In  the 
centre  a  strand  of  them  is  marked  off  from  the  rest  by 
their  small  size  and  in  some  cases  by  their  altered  cell 
walls ;  here  we  have  the  first  indication  of  a  conducting 
system  in  the  land  plant.  The  leaves  are  flat  plates  of 


EVOLUTION  OF  THE  LAND  PLANT,,,  ;*qj 

cells.  When  fertilisation  has  been  effected  the  ovum 
becomes  clothed  with  a  cell  wall  and  develops  to  form  the 
sporocarp.  This  is  a  small  ovoid  body  which  is 
formed  at  the  end  of  a  long  stalk-like  structure 
which  grows  out  of  the  archegonium.  It  re- 
mains attached  in  this  loose  way  to  the  arche- 
gonium and  so  appears  to  grow  out  of  the 


FIG.  38.  Section  of  apex  of  stem  of  moss  bearing 
antheridia. 

ordinary  moss  plant.  The  sporocarp  is  rather 
complex  in  its  structure  (Fig.  40).  It  is  not 
all  devoted  to  the  formation  of  spores,  but 
contains  a  great  deal  of  nutritive  tissue,  so 
that  it  is  capable  of  living  for  a  long  time.  In 

,/IG-  p-    its  lower  part  it  develops  some  chlorophyll- 
Moss  plant.  ,     .     .  ,,  ,,      . r  .  .  r      , 

containing  cells,  so  that  it  can  manufacture 
its  own  food.  The  spores  are  developed  in  particular 
bands  of  cells  which  arise  in  the  interior  and  which  are 
usually  found  in  the  form  of  a  hollow  cylinder  surround- 


IO2 


BOTANY 


ing  a  central  core  of  ordinary  cells  (Fig.  40  C,  s).  The 
moss  sporocarp  in  most  cases  is  provided  with  a  special 
mechanism  to  cause  it  to  open 
when  the  spores  are  ripe  so  that 
u  the  latter  may  be  discharged. 


FIG.  40.  Funaria.  A,  young  sporo- 
carp ;  c,  capsule  or  sporogonium.  B, 
moss  plant  with  attached  sporocarp 
mature;  s,  stalk  of  sporogonium ;  /, 
capsule;  g,  leaves  of  the  moss  plant. 
C,  section  of  a  sporogonium ;  s,  layer 
of  cells  which  develop  the  spores. 

,-,  -r>  When    the   spore    germinates 

,  it  produces  a  little  filamentous 
FIG.  39-  ,4,  section  of  stem  of    __£_     __Al_  ___^_._^  ,_„  __^__  x___.,__ 


moss  bearing  archegonia ;  B,  outgro wth  which  branches  freely 

open  neck  of  archegonium ;    on     the     moist    Soil,    and    which 

resembles  very  closely  a  fila- 
mentous alga.  The  moss  plants 
originate  by  the  development  of  buds  upon  this  out- 
growth, which  is  known  as  a  protonema. 


C,  archegonium  highly  mag- 
nified.    (After  Sachs.) 


EVOLUTION  OF  THE  LAND  PLANT       103 

The  altered  conditions  of  their  life  can  now  be  seen  to 
lead  to  a  very  great  change  in  the  life  history  of  plants. 
The  aquatic  forms  with  their  comparatively  simple  body 
and  the  process  of  fertilisation  depending  on  the  power 
of  the  sperm  to  swim  to  the  ovum  proved  exceedingly 
unsuitable  for  life  on  land,  and  very  soon  a  great  develop- 
ment of  the  sporocarp  began,  and  it  gradually  assumed 
the  form  of  a  self-supporting  plant — the  sporophyte. 
From  this  point  upward  the  tendency  of  evolution  was 
to  diminish  the  plant  which  bore  the  ova  and  sperms  till 
there  was  hardly  anything  of  it  except  these  repro- 
ductive cells,  and  at  the  same  time  to  make  the  sporo- 
phyte more  and  more  important,  till  it  became  far  the 
most  highly  developed,  and  of  infinitely  larger  size  than 
what  was  left  to  represent  the  original  ancestor. 

We  find  this  change  well  established  in  the  ferns  and 
the  plants  which  are  allied  to  them.  The  ova  and 
sperms  are  produced  on  a  thin  plate-like  body  of  about 
half  an  inch  in  diameter.  This  lies  upon  the  soil  and  is 
attached  to  it  by  rhizoids.  The  antheridia  and  arche- 
gonia  are  borne  upon  its  lower  surface  and  fertilisation 
is  accomplished  by  free-swimming  sperms.  The  plant 
is  known  as  the  prothallus  ;  it  is  made  up  of  cells  which 
are  much  alike  throughout.  The  sporocarp  arising  from 
it  has  been  replaced  by  a  large  plant,  showing  root,  stem, 
and  leaves.  We  do  not  know  the  stages  by  which  so 
large  and  well-organised  a  structure  has  been  developed 
from  the  original  sporocarp,  but  it  represents  the  latter 
in  its  place  in  the  life  history  of  the  fern.  So  important 
has  it  become,  and  so  great  in  comparison  with  the 
prothallus,  that  it  has  come  to  be  called  the  "  plant," 
leaving  the  ovum-bearing  plant  to  be  regarded  as  the 
"  prothallus  of  the  fern." 

We  see  that  with  the  establishment  of  terrestrial 
habit,  the  life  history  of  the  plants  thus  became  quite 
revolutionised.  The  large  tree  of  the  land  flora  does 


104  BOTANY 

not  correspond  with  the  large  seaweed,  but  with  a  parti- 
cular reproductive  structure  to  which  the  latter  gave  rise. 
The  fern  plant  differs  a  good  deal  from  the  seed-bear- 
ing plants  which  we  have  examined  in  the  earlier 
chapters.  It  has-  as  a  rule  only  underground  stems 
which  are  known  as  rhizomes;  each  bears  very  few 
leaves  at  or  near  its  apex.  The  stem  grows  horizontally 
under  the  surface  of  the  ground,  reacting  to  gravitation 


FIG.  41.  Prothallus  of  fern.     X5- 

in  a  way  unlike  either  stem  or  root  of  the  flower- 
ing plant.  The  leaves  emerge  from  the  soil  generally 
rolled  up  in  the  form  of  a  shepherd's  crook,  in  conse- 
quence of  the  great  growth  of  the  under  surface.  They 
soon  straighten  themselves  as  the  growth  of  the  upper 
surface  becomes  vigorous,  just  as  in  the  case  of  the  leaf 
in  the  bud  of  the  flowering  plant.  They  are  then  found 
to  be  very  much  divided,  except  in  a  few  cases.  Roots 
are  given  off  from  the  rhizome  in  large  numbers. 

The  structure  of  the  fern  rhizome  differs  in  detail 
from  that  of  either  type  of  flowering  plant  we  have 


EVOLUTION  OF  THE  LAND  PLANT       105 

described.  There  are  the  three  systems  of  tissue, 
dermatogen,  periblem,  and  plerome,  but  they  are  not  so 
clearly  distinguishable.  They  originate  by  divisions  in 
a  single  large  pyramid-shaped  cell  at  the  apex — the  so- 
called  apical  cell,  which  cuts  off  segments  of  itself  by 
walls  parallel  in  turn  to  each  of  its  sides  except  the 
outside  one.  These  segments  divide  later  to  make  up 
the  mass  of  cells  at  the  end,  in  which  the  differentiation 
into  the  three  regions  spoken  of  takes  place. 

The  plerome  does  not  give  rise  to  a  single  solid  or 
hollow  cylinder,  but  to  one  in  which  the  conducting 
strands  form  a  cylindrical  network.  The  strands  to 
the  leaves  leave  the  network  at  the  margins  of  the 
meshes.  The  strands  are  composed  of  wood  and  bast 
as  in  other  cases,  but  in  their  arrangement  the  bast 
usually  surrounds  the  wood  completely.  There  is  no 
cambium  and  no  increase  in  thickness  occurs. 

The  leaves  are  something  like  the  leaves  of  a  dicoty- 
ledon in  structure,  but  most  of  the  internal  tissue  re- 
sembles the  spongy  tissue  rather  than  the  palissade. 
The  epidermis  contains  chloroplasts. 

The  structure  of  the  root  resembles  that  of  a  dicoty- 
ledon very  closely,  but  there  is  no  provision  for  any 
increase  in  thickness.  The  pericycle  is  several  layers  of 
cells  in  thickness. 

The  fern  bears  its  spores  in  little  cases  known  as 
sporangia.  In  our  common  ferns  these  are  grouped  to- 
gether on  the  under  sides  of  the  leaves  in  little  patches  called 
sori  (Fig.  42).  Each  sporangium  contains  a  number  of 
spores.  Each  spore  on  germination  produces  a  prothallus. 

We  found  it  impossible  to  say  how  such  a  plant  as 
this  has  come  to  be  formed  in  the  place  of  the  sporocarp 
of  the  mosses.  We  find  almost  as  great  difficulty  in 
tracing  the  formation  of  the  flowering  plants  from 
plants  having  the  degree  of  development  of  the  ferns. 

We  may  well  realise  that  with  the  gradual  increase  of 


106  BOTANY 

complexity  of  the  sporocarp  it  came  to  have  an  inde- 
pendent existence,  and  that  when  it  had  achieved  this, 
it  became  an  erect  plant,  the  conditions  we  have  already 
alluded  to  making  this  necessary.  At  some  stage  or 
other  in  the  development  a  difference  arose  among  the 
prothalli  arising  from  the  spores,  some  giving  rise  to 
sperms  only  while  the  others  only  produced  ova.  Gradu- 
ally the  change  spread  to  the  spores  themselves,  those 
giving  rise  to  sperm-bearing  prothalli  remaining  small, 


FIG.  42.  Section  of  sorus  of  fern  showing  sporangia 
covered  by  an  indusium.     X5o.     (After  Kny.) 

those  producing  ova-bearing  prothalli  becoming  much 
larger.  The  prothalli  also  changed.  Those  from  the 
small  spores  became  in  some  cases  filamentous,  and  in 
all  extremely  minute,  consisting  of  little  more  than  the 
organ  giving  rise  to  the  sperms.  Those  from  the  large 
spores  only  protruded  partly  from  the  ruptured  spore, 
and  came  to  be  developed  almost  entirely  inside  it,  the 
spore  splitting  its  coats  but  slightly.  Such  prothalli 
developed  very  few  archegonia. 

These    forms    are    still    represented    to-day    by    the 
Selaginellas,  a  group  classed  among  the  fern-like  plants. 


EVOLUTION  OF  THE  LAND  PLANT   107 


As  the  spore-bearing  plant  or  sporophyte  grew  larger 
and  by  its  erect  position  became  carried  away  from  the 
ground,  the  separation  of  the  two  kinds  of  prothalli 
developed  from  the  spores  rendered  fertilisation  by 
means  of  free-swimming  sperms  increasingly  difficult. 
The  larger  spores, 
too,  tended  to  stay 
longer  and  longer  nrch 
in  their  sporangia, 
so  that  the  time  for 
fertilisation  became 
shortened.  This  led 
to  the  establish- 
ment of  a  new 
method  of  securing 
fertilisation  which 
we  find  exhibited 
by  the  flowering 
plants.  The  pro- 
thalli -  bearing  ova 
became  entirely  en- 
closed in  the  large 
spore,  and  those 
bearing  sperms  be- 
came entirely  fila- 
mentous. Instead 
of  the  sperm 
travelling  to  the  ovum,  the  small  spore  itself  was  brought 
by  various  ways  to  the  immediate  neighbourhood  of  the 
large  one,  so  that  the  prothalli  were  developed  in  close 
proximity  to  each  other.  The  filamentous  prothallus  of  the 
small  spore  bored  its  way  into  the  surroundings  of  the  large 
spore  and  the  latter  remained  altogether  in  its  sporangium. 
Fertilisation  consequently  came  about  by  means  of  a 
tubular  outgrowth  of  the  little  spore,  instead  of  a  free- 
swimming  sperm.  If  we  compare  the  two  methods,  both 


FIG.  43.  Germination  of  megaspore  of  Selagi- 
nella,  showing  prothallus  almost  entirely 
inside  the  spore,  which  has  opened  at  the 
apex,  arch,  archegonia;  oos,  ovum;  em, 
young  embryos  at  different  stages  of  de- 
velopment. 


io8  BOTANY 

of  which  still  persist,  we  find  that  in  the  latter  all  the 
structures  are  exposed  freely ;  in  the  former  the  greater 
part  are  embedded  deeply  in  other  parts  of  the  plant. 

We  shall  return  to  this  subject  in  connection  with 
the  mechanism  of  the  flower  in  the  higher  plants. 


CHAPTER  XIV 

REPRODUCTION    OF    FLOWERING    PLANTS — 
VEGETATIVE   PROPAGATION 

THE  last  requirement  of  the  plant  we  must  consider  is 
the  power  of  reproducing  its  species. 

There  is  a  good  deal  of  variety  in  the  ways  in  which 
this  is  possible ;  some  methods  consist  in  separating 
certain  parts  of  the  ordinary  plant  body  which  after  a 
time  grow  into  new  plants ;  in  others  special  reproduc- 
tive cells  are  produced.  The  first  method  is  often 
spoken  of  as  vegetative  propagation.  The  parts  which 
can  be  spared  for  it  in  different  plants  are  a  good  deal 
modified  and  have  come  to  be  considered  as  separate 
organs.  They  include  modified  stems,  leaves,  or  roots. 

We  find  such  structures  include  a  part  which  is  cap- 
able of  growing — some  kind  of  bud — and  a  store  of  food 
for  the  nutrition  of  the  shoot  which  arises  from  it. 

The  most  easily  observed  of  the  modified  stems  is 
known  as  the  tuber.  This  is  a  stem  or  branch  which 
grows  under  the  ground  instead  of  above  the  surface. 
It  consists  of  a  few  internodes,  which  become  greatly 
swollen  and  filled  with  starch  and  protein.  The  leaves 
are  only  to  be  seen  with  the  help  of  a  lens;  they 
are  minute  scales,  which  never  develop  further.  The 
buds  are  in  their  axils  and  are  generally  several  in 
number,  so  that  a  tuber  can  put  out  several  shoots,  each 
of  which  may  grow  into  a  new  plant.  The  most  familiar 
instance  of  a  tuber  is  afforded  by  the  common  potato. 


REPRODUCTION  109 

Another  form  of  underground  stem  which  may  be 
included  here  is  the  rhizome  of  such  plants  as  the  iris. 
It  grows  partly  under  the  surface  of  the  ground,  but  its 
upper  side  often  protrudes  and  becomes  green.  It 
never  grows  vertically  into  the  air,  but  its  terminal  bud 
sends  a  shoot  upwards  which  bears  the  foliage  leaves 
and  flowers.  It  is  swollen  and  filled  with  food  like  the 
potato,  but  it  does  not  become  detached  as  the  tuber 
does.  It  is  in  fact  the  main  stem  of  the  plant  and  pro- 
duces only  a  single  bud,  instead  of  a  number. 

Two  other  forms  of  propagating  organ  are  the  bulb 
and  the  corm.  The  bulbs  are  very  large  buds,  a  rela- 
tively small  conical  stem  being  covered  with  a  large 
number  of  scaly  leaves,  the  inner  ones  becoming  very 
succulent  and  containing  food,  chiefly  sugar,  for  the 
young  plant,  into  which  the  growing  apex  will  develop. 
The  outer  leaves  remain  dry  scales  and  are  only  pro- 
tective to  the  succulent  interior.  The  onion  is  a  good 
example  of  the  bulb. 

The  corm  consists  of  a  few  internodes  of  an  under- 
ground stem.  It  is  solid  but  resembles  the  bulb  in 
being  clothed  with  dry  scaly  leaves.  There  is  a  bud  at 
its  apex  which  is  like  the  most  internal  part  of  the  bulb 
of  the  onion.  An  example  is  afforded  by  the  crocus. 

Roots  which  are  modified  for  reproductive  purposes 
are  shown  by  the  dahlia.  They  swell  and  store  food 
after  the  manner  of  the  tuber,  but  they  do  not  develop 
buds.  They  give  up  their  contents  to  a  shoot  put  out 
from  a  portion  of  the  stem  of  the  original  plant. 

Vegetative  propagation  is  very  commonly  employed 
by  gardeners,  by  means  of  cuttings.  A  piece  of  a  young 
stem,  with  a  few  leaves  and  buds,  cut  from  the  parent 
will,  when  planted,  often  develop  roots  from  the  cut 
surface  and  so  establish  a  new  plant.  Some  plants  pro- 
pagate themselves  naturally  in  this  way ;  any  detached, 
injured  portion,  when  left  upon  damp  soil,  will  put  out 


no  BOTANY 

adventitious  roots  and  so  grow  into  a  plant.  The  power 
of  putting  out  adventitious  roots  is  often  used  by  plants 
for  this  purpose.  Other  familiar  examples  are  afforded 
by  the  runners  of  the  strawberry,  the  suckers  of  the 
raspberry,  the  stolons  of  the  gooseberry  and  other  plants. 

The  runner  is  a  lateral  branch  which  grows  along  the 
surface  of  the  ground  and  puts  out  adventitious  roots 
at  its  nodes.  In  the  case  of  the  strawberry  each  runner 
usually  has  two  nodes,  one  terminal  and  one  half  way 
along  its  length.  Two  new  plants  arise  on  each  in  con- 
sequence, and  when  they  are  established  the  internodes 
connecting  them  with  the  parent  die. 

The  stolon  is  much  like  the  runner ;  arising  as  a  lateral 
branch  above  the  ground  it  bends  down  and  on  reaching 
the  soil  it  puts  out  adventitious  roots  and  becomes 
detached  from  the  parent.  The  sucker  arises  from  the 
underground  portion  of  the  stem  and  growing  horizon- 
tally for  a  time  ultimately  turns  upwards  and  emerges 
into  the  air — becoming  then  detached  as  in  other  cases. 

These  methods  are  purely  vegetative  and  do  not 
involve  the  production  of  any  form  of  specialised  repro- 
ductive cell. 

CHAPTER  XV 

THE    INFLORESCENCE    AND    THE    FLOWER 

As  we  traced  the  development  of  the  reproductive 
organs  from  the  seaweeds  upwards  we  found  that  two 
special  kinds  have  become  constant,  the  ova  and 
sperms  on  the  one  hand  and  the  spores  on  the  other. 
We  also  learned  that  the  plant  form  which  produces  the 
spores  has  become  the  conspicuous  plant,  with  its  roots, 
stems,  and  leaves ;  while  those  which  give  rise  to  the  ova 
and  sperms  have  dwindled  away  in  dimensions  till  they 
no  longer  have  an  existence  apart  from  the  spore  itself. 
We  will  study  first  the  reproductive  processes  of  the 


THE  INFLORESCENCE  AND  THE  FLOWER     in 

form  which  produces  spores — the  so-called  sporophyte 
phase  of  the  plant's  life  history. 

We  have  pointed  out  that  in  the  flowering  plants  two 
kinds  of  spore  are  produced.  The  little  ones  resemble 
the  spores  of  the  ferns  and  mosses ;  they  are  small  cells 
with  thick  walls,  usually  spherical  in  shape.  They  are 
liberated  from  the  sporangia  in  which  they  are  de- 
veloped and  germinate  after  their  removal.  The  large 
spores  are  produced  singly  in  each  sporangium  and 
never  escape  from  it.  The  sporangium  is  a  bulky  struc- 
ture and  consequently  the  spore  need  not  develop  the 
thick  wall  found  on  the  little  spore. 

We  have  now  to  see  where  these  spores  occur  and 
what  is  the  story  of  their  behaviour. 

In  most  ferns  we  saw  that  the  sporangia  occur  in 
patches  on  certain  places  on  the  ordinary  leaves.  In 
some,  however,  they  are  found  on  very  specialised  out- 
growths, generally  .spoken  of  as  modified  leaves.  In  the 
flowering  plant  the  sporangia  are  usually  collected  at 
the  end  of  special  shoots  which  are  known  as  flowers. 
A  special  system  of  the  branching  is  set  apart  for  their 
production.  It  may  consist  of  very  many  branches  and 
may  be  very  complicated,  hence  it  is  usually  considered 
separately  under  the  name  of  the  inflorescence.  Its 
ultimate  branches  are  known  as  flowers,  the  leaves  in 
whose  axils  they  are  found  are  termed  bracts. 

A  very  common  form  of  the  inflorescence  is  that  in 
which  the  main  axis  continues  to  increase  in  length  and 
bears  a  succession  of  flowers  as  it  grows  on,  so  that  the 
youngest  flower  is  nearest  the  apex.  This  corresponds 
to  the  indefinite  method  of  vegetative  branching.  The 
inflorescence  is  called  a  raceme.  There  are  many  modi- 
fications of  it  on  all  sides :  if  the  flowers  have  no  stalks 
it  is  called  a  spike;  if  they  all  reach  the  same  level 
through  the  older  stalks  growing  longer  than  the  younger 
it  is  a  corymb  ;  if  the  axis  is  so  short  that  all  the  flowers 


U2  BOTANY 

appear  to  start  at  the  same  point  it  is  an  umbel  (Fig.  44) ; 
if  it  is  itself  branched  it  becomes  a  panicle. 

A  fundamentally  different  form  is  that  in  which  the 
main  stalk  is  at  once  terminated  by  a  flower  and  other 
stalks  grow  out  from  under  it  to  bear  similarly  a  flower 


FIG.  44.  Umbellate  inflorescence  of  ivy.     (After  Marshall  Ward.) 

at  the  end  of  each ;  we  have  here  what  is  called  a  cyme. 
It  corresponds  to  the  definite  method  of  branching  of 
the  vegetative  parts.  As  in  the  case  of  the  raceme 
there  are  many  varieties  of  this  kind  of  inflorescence. 

A  very  noticeable  modification  of  the  raceme  is  the 
form  in  which  the  apex  is  not  elongated  or  conical,  but 
spreads  out  into  a  flat  receptacle  on  which  large  numbers 
of  small  flowers  or  florets  are  arranged  so  that  the 
youngest  are  in  the  centre  and  the  oldest  ones  round  the 


THE  INFLORESCENCE  AND  THE  FLOWER     113 

circumference.  This  is  known  as  a  capitulum.  It  is 
generally  surrounded  by  a  number  of  bracts,  which  form 
what  is  called  an  involucre. 

In  its  most  primitive  form  the  flower  probably  con- 
sisted of  an  axis  on  which  the  sporangia  were  borne,  and 
the  latter  were  of  only  one  kind.  In  many  cases,  most 
likely  arising  later,  we  find  this  axis  bearing  two  sporan- 
gial  series  of  outgrowths,  one  for  the  production  of  each 
kind  of  spore,  and  this  arrangement  gradually  became 
widespread,  the  large  spores  being  developed  in  the 
series  nearest  the  apex  and  the  small  ones  in  the  lower 
of  the  two.  As  each  series  stood  in  a  circle  round  the 
axis,  all  its  parts  arising  at  the  same  node,  they  received 
the  name  of  whorls. 

In  the  more  perfect  forms  of  flower  which  have  been 
developed  as  time  has  gone  on  (Fig.  45)  we  find,  besides 
these  two  whorls  of  spore-bearing  leaves,  two  other 
whorls  growing  below  them.  These  two  constitute  the 
perianth  of  the  flower.  A  leaf  lower  down  still,  in  the 
axil  of  which  the  flower  arises,  is  known  as  a  bract. 

The  perianth  of  the  flower  is  formed  then  by  two 
series  or  whorls  of  leaves.  The  outer  ones  are  green 
and  often  sturdy  in  their  texture,  and  they  protect 
the  young  flower  while  it  is  in 
the  condition  of  a  bud.  They 
are  known  as  sepals,  and  the 
collection  of  them  forms  the 
calyx.  The  inner  whorl  is 
usually  made  up  of  highly 
coloured  leaves  which  serve  to 
make  the  flower  conspicuous. 
They  are  called  petals,  and  the  FlG'  45'  ' 
collection  is  termed  the  corolla. 
These  whorls  frequently  have  their  sepals  or  petals  joined 
together  for  part  or  even  the  whole  of  their  length. 

The  whorls  which  bear  the  sporangia  are  distinguished 


H4  BOTANY 

from  the  rest  as  the  sporophylls.  They  are  usually 
considered  to  be  modified  leaves,  as  the  name  indicates. 
The  outer  whorl  consists  of  bodies  very  unlike  leaves  in 
appearance.  Each  shows  a  slender  stalk  or  filament 
bearing  at  its  top  a  swollen  head  or  anther.  The  whole 
club-shaped  organ  is  called  a  stamen.  Each  anther 
contains  a  group  of  four  chambers,  which  are  the 
sporangia,  and  inside  them  are  the  small  spores.  They 
are  commonly  spoken  of  as  pollen  sacs,  containing  pollen 
grains.  These  were  the  old  names  applied  to  them 
before  their  true  nature  was  understood,  and  they  are 
retained  still  as  a  matter  of  convenience. 

Like  the  sepals  and  petals  the  stamens  may  be  joined 
together  in  various  ways,  or  they  may  be  free.  They 
may  grow  from  the  flower  stalk,  or  they  may  appear  to 
spring  from  the  calyx  or  the  corolla.  These  appearances 
are  due  to  curious  irregularities  in  the  growth  of  the 
parts  of  the  flower. 

The  sporophylls  of  the  final  whorl  are  known  as 
carpels,  and  together  they  form  the  pistil.  In  shape  the 
carpels  resemble  a  leaf  folded  on  the  mid-rib  till  the 
edges  meet  and  become  united,  so  that  a  cavity  is 
formed  inside  them.  This  cavity  is  the  ovary.  It  is 
more  usual  to  find  the  carpels  united  by  their  edges  to 
form  a  large  ovary,  which  is  often  divided  up  into  several 
chambers.  The  sporangia  are  found  in  the  interior  of 
the  ovary,  attached  usually  to  the  edges  of  the  carpels. 
They  were  formerly  called  ovules,  and  the  name  is  still 
often  used  (Fig.  46) .  The  ovule  or  mega-sporangium  is  a 
fairly  substantial  structure.  As  the  spore  never  leaves 
it  but  produces  its  prothallus  in  its  own  interior  while 
still  within  it,  and  as  the  young  embryo  plant  is  developed 
on  the  prothallus  inside  the  spore,  it  is  clearly  an  organ 
of  the  greatest  importance.  It  consists  of  a  mass  of 
small  cells  called  the  nucellus,  and  is  covered  by  two 
membranes  or  integuments  which  also  are  many-layered 


THE  INFLORESCENCE  AND  THE  FLOWER     115 


and  strong.  At  its  upper  end  the  integuments  do  not 
cover  it  but  leave  a  little  aperture,  the  micropyle.  Each 
ovule  contains  a  single  thin-walled  spore,  which  often 
occupies  a  very  large  space  in  its  interior.  It  is  often 
called  the  embryo  sac, 
from  the  fact  that  the 
embryo  is  developed  in 
its  interior. 

The  ovary  is  only  the 
basal  part  of  the  carpel 
or  the  pistil.  There  is 
always  a  sticky  apex  to 
it,  which  is  the  stigma. 
The  stigma  is  usually 
placed  at  the  top  of  an 
elongated  part  of  the 
pistil,  seeming  to  arise 
from  the  top  of  the  ovary 
—this  is  the  style. 

The  pistil  is  always  the 
terminal  whorl  of  the 
flower.  Some  flowers, 
however,  owing  to  a 
curious  mode  of  growth 
of  the  axis,  which  be- 
comes concave  and  comes  to  surround  and  often  to  cover 
in  the  carpels,  appear  to  bear  their  stamens  and  perianth 
above  the  ovary.  The  latter  is  then  termed  inferior. 

Many  flowers  are  not  provided  with  all  these  parts. 
It  is  not  at  all  uncommon  to  find  that  the  corolla  is  not 
developed.  In  such  cases  it  frequently  happens  that 
the  calyx  is  not  green  but  brightly  coloured.  In  many 
monocotyledons  both  calyx  and  corolla  are  coloured  so 
similarly  that  it  is  difficult  to  distinguish  between  them. 
The  flowers  of  some  of  our  forest  trees  do  not  possess  a 
perianth  at  all,  and  in  the  flowers  of  others  only  one 


Fig.  46.  Section  of  an  ovule,  mac, 
megaspore  or  embryo -sac;  oos, 
ovum;  pt,  pollen  tube. 


n6 


BOTANY 


series  of  sporophylls  is  present.  Those  which  bear 
stamens  only  are  known  as  staminate  flowers ;  those 
which  possess  only  carpels  are  called  pistillate.  Such 
flowers  are  generally  very  small  and  inconspicuous. 

Another  type  of  flower  altogether  is  found  in  the 
group  of  plants  called  Gymnosperms,  which  is  repre- 
sented in  this  country  by  the  fir  trees  and  their  allies. 


FIG.  47.  A,  twig  of  fir  tree  bearing  a  young  female  cone;  B,  twig  bearing 
several  male  cones;  C,  ovaliferous  scale  from  A  showing  two  ovules 
on  the  under  surface.  (After  Scott.) 

In  most  of  these  the  sporophylls  are  arranged  in  a  close 
spiral  round  an  axis  and  form  the  structures  known  as 
cones.  The  fir  tree  itself  bears  two  kinds  of  these  cones 
(Fig.  47)  each  bearing  one  kind  of  spore.  The  smaller 
cones  are  composed  of  a  large  number  of  very  small 
leaves  arranged  spirally  round  the  axis  of  the  flower, 
covering  or  overlapping  each  other  very  closely.  On 
the  back  of  each  leaf  or  sporophyll  are  two  sporangia,  or 
pollen  sacs,  each  containing  a  large  number  of  micro- 
spores  or  pollen  grains.  Larger  cones  are  developed  in 
connection  with  the  production  of  the  megaspores.  The 


POLLINATION  117 

general  arrangements  of  the  cones  are  similar  to  those 
of  the  smaller  ones.  There  is  a  central  axis  round  which 
the  leaves  or  sporophylls  are  spirally  arranged.  Each 
sporophyll  has  on  its  inner  face  a  flattened  outgrowth 
which  in  some  cases  becomes  larger  than  the  sporophyll 
itself.  On  its  upper  side  this  so-called  ovali/erous  scale 
bears  two  sporangia  or  ovules.  There  is  no  closing  up 
of  the  sporophyll  to  form  an  ovary — hence  the  name  of 
the  group  gymnosperm,  meaning  naked  seed.  Each  ovule 
has  much  the  same  structure  as  that  described  above,  but 
there  is  only  one  integument  and  the  micropyle  is  larger. 


CHAPTER  XVI 

POLLINATION    AND    ITS    MECHANISMS — FERTILISATION 

THE  separation  of  the  spores  and  the  diminution  of  the 
phase  of  the  plant  which  results  from  their  germination, 
consisting  of  little  more  than  the  ova  and  the  sperms, 
have  made  it  impossible  for  fertilisation  to  take  place  by 
means  of  free-swimming  sperms.  To  ensure  its  occur- 
rence it  has  become  necessary  to  secure  that  the  two 
spores  producing  ova  and  sperms  respectively  shall  be 
brought  close  to  each  other,  so  that,  when  they  germinate, 
their  prothalli,  now  so  rudimentary,  shall  be  able  to 
meet,  in  order  that  the  sperm  may  make  its  way  to  the 
ovum.  This  has  been  effected  by  the  transport  of  the 
pollen  grain — the  small  spore — to  that  part  of  the  flower 
which  bears  the  large  spore  or  embryo  sac.  This  trans- 
ference of  the  pollen  is  spoken  of  as  pollination.  In  the 
fully  organised  flowers  of  Dicotyledons  and  Monocoty- 
ledons the  pollen  grain  is  deposited  on  the  stigma — in 
the  cones  of  the  Gymnosperms  it  reaches  the  ovule  or 
megasporangium  itself.  The  problem  of  pollination 
presents  many  interesting  features  which  we  may  now 
briefly  examine. 


n8  BOTANY 

As  in  each  perfect  flower  both  spores  are  produced,  the 
problem  at  first  sight  appears  to  involve  only  the  transfer- 
ence of  its  pollen  grains  to  its  stigma.  In  many  cases  this 
is  all  that  takes  place,  but  simple  as  the  method  is,  its 
occurrence  is  the  exception  rather  than  the  rule,  for  much 
stronger  and  healthier  plants  -are  yielded  when  the  pollen 
from  one  flower  is  made  to  fall  upon  the  stigma  of  another. 
The  former  simpler  process  is  termed  self-pollination;  it 
was  probably  the  most  primitive,  but  became  gradually 
superseded  by  the  latter,  known  as  cross-pollination. 

The  necessity  for  pollination  and  the  advantages 
presented  by  the  crossing  have  led  to  the  development 
of  various  mechanisms  in  flowers  to  bring  it  about. 
Here,  perhaps  more  than  in  any  other  case,  do  we 
recognise  the  difficulties  entailed  by  the  stationary 
situation  of  the  plants.  The  small  spores  though  set  free 
are  non-motile  and  must  be  carried  to  the  stigma  by 
some  external  agency  which  is  not  under  the  control  of 
either  plant.  Hence  it  is  that  we  find  perhaps  more 
adaptations  or  modifications  of  structure  in  connection 
with  this  function  than  any  other. 

The  transport  of  the  pollen  at  the  outset  must  have 
been  brought  about  by  the  physical  agents  of  nature, 
water  in  the  case  of  aquatic  flowering  plants,  wind  in 
that  of  those  of  terrestrial  habit.  In  both  these  cases  the 
chances  of  the  liberated  pollen  being  floated  or  blown  to 
the  stigma  are  not  great.  Hence  the  pollen  in  such 
flowers  is  always  produced  in  large  quantities.  Aerial 
transport  is  further  aided  by  the  production  of  very 
light,  dry,  relatively  smooth  pollen,  in  some  cases  aided 
by  mechanical  modifications  of  the  wall,  expansion 
into  bladders,  etc.  The  stigmas  in  plants  which  are 
pollinated  in  this  way  are  often  divided  a  good  deal, 
becoming  in  the  case  of  the  grasses  quite  feathery,  so  as 
to  offer  as  much  surface  as  possible  to  entangle  the 
pollen.  The  surface  of  the  stigma  often  bears  a  velvet- 


POLLINATION  119 

like  pile  of  short  hairs,  and  usually  secretes  a  sticky, 
sugary  excretion — features  which  subserve  the  same  end. 

This  so-called  anemophilous  pollination  is  uncertain 
and  wasteful  and  has  in  many  cases  been  superseded  by 
the  intervention  of  insects.  Hence  we  find  developed 
the  colour,  fragrance,  and  other  attractions  which 
flowers  so  generally  possess.  Colour  and  fragrance  cause 
conspicuousness,  and  appeal  to  insect  visitors.  The 
latter  seek,  of  course,  more  substantial  benefits  than 
these,  regarding  the  flowers  as  the  seats  of  supplies  of 
nutritive  substances  which  they  require.  While  they 
rifle  the  flowers  of  both  pollen  and  honey,  they  inad- 
vertently serve  their  turn  by  transferring  the  pollen 
from  the  stamens  of  one  flower  to  the  stigma  of  another. 

The  relations  that  have  thus  come  to  be  so  widely 
established  have  probably  grown  up  very  gradually  and 
in  an  infinite  variety  of  ways.  Different  insects  visit 
different  flowers  and  the  influence  of  the  particular  kind 
of  insect  visitor  explains  the  manner  of  modification 
which  the  particular  flower  has  undergone.  The  modi- 
fications are  almost  infinite  in  variety  and  we  can  only 
here  deal  with  them  in  a  very  general  way. 

The  earliest  modifications,  which  were  only  slight, 
were  associated  with  the  discovery  by  certain  lowly 
insects  that  pollen  itself  formed  nutritious  food.  The 
flower  was  widely  open,  the  perianth  leaves  spreading 
symmetrically  round  the  axis  below  the  stamens,  and 
the  latter  opened  and  let  their  pollen  fall.  A  slight 
change  of  colour,  probably  from  green  to  yellow  or 
white,  made  the  flowers  sufficiently  conspicuous,  and 
the  visits  of  the  insects  followed. 

But  a  further  attraction  was  afforded  later  by  the 
formation  and  storage  of  honey  in  the  flower.  It  is 
impossible  to  discuss  this  subject  in  detail  as  it  has 
exhibited  such  infinite  variety  of  modification.  The 
storage  of  honey  led  to  the  development  of  pocket-like 


120  BOTANY 

bodies,  slippers,  or  spurs,  either  on  the  receptacle  or  the 
perianth  leaves.  Irregularity  of  form  of  the  calyx  or 
corolla  was  thus  caused.  Markings  on  the  individual 
sepals  or  petals  followed,  which  directed  the  attention  of 
the  insect,  usually  a  wasp  or  bee,  to  the  hidden  store. 
The  honey  became  hidden  in  such  a  way  that  to  reach  it 
the  visitor  brought  some  definite  part  of  its  person  into 
contact  with  the  stamens,  and  on  visiting  another 
flower  touched  the  stigma  with  the  same  region.  The 
influence  of  colour  and  of  fragrance  were  brought  to  bear 
upon  the  insect  visitors  with  similar  results,  the  whole 
mechanism  of  any  particular  flower  being  correlated 
with  the  habit  of  some  appropriate  insect. 

The  mechanisms  of  some  flowers  at  the  present  time 
are  even  more  complete  than  this.  As  cross-pollination 
is  preferable  to  self-pollination,  the  flower  is  adapted 
to  some  visitor  to  secure  it;  but  as  self-pollination  is 
better  than  none  at  all,  if  the  insect  mechanism  should 
fail  to  act,  the  stigma  is  in  some  way  brought  into 
contact  with  the  stamens  of  the  same  flower. 

The  apparent  ease  with  which  self-pollination  of 
almost  every  flower  can  be  brought  about  has  led  in 
many  cases  to  a  peculiar  modification  to  secure  the 
advantages  of  crossing.  This  consists  in  the  maturing 
of  the  stamens  and  the  stigmas  of  a  flower  at  different 
times,  so  that  if  self-pollination  should  take  place  there 
would  be  no  result  therefrom.  This  condition  is  known 
as  dichogamy ;  flowers  whose  stamens  mature  before 
their  carpels  are  said  to  be  protandrous ;  those  in 
which  the  condition  is  reversed  are  proterogynous. 

It  is  of  course  obvious  that  cross-pollination  is  the 
only  possible  method  in  the  cases  of  those  plants  which 
bear  pistillate  and  staminate  flowers. 

The  cones  of  the  fir  trees  are  pollinated  by  wind. 
When  the  cone  is  ripe  its  leaves  separate  a  little  from 
each  other  so  that  the  pollen  grains,  blown  in  large 


POLLINATION  121 

numbers,  can  be  carried  down  into  the  heart  of  the  cone 
to  the  bases  of  the  scales  on  which  the  ovules  are  seated, 
as  already  described.  There  is  no  stigma  and  the 
ovules  are  exposed;  hence  the  pollen  grain  is  dropped 
upon  the  micropyle  of  the  ovule,  down  which  it  is 
drawn  into  a  little  space  just  above  the  body  of  the 
ovule  itself  and  inside  its  integument.  This  little 
cavity  is  known  as  the  pollen  chamber. 

In  all  cases  these  mechanisms  bring  the  two  spores 
very  near  to  one  another.  In  the  Gymnosperms  they 
are  separated  by  only  the  substance  of  the  upper  part  of 
the  ovule,  whose  spore  or  embryo  sac  lies  quite  near  the 
upper  end.  In  the  other  flowering  plants  the  two 
spores,  the  pollen  grain  and  the  embryo  sac,  are  separ- 
ated by  the  length  of  the  style,  the  chamber  of  the 
ovary,  and  the  upper  part  of  the  ovule. 

The  next  steps  in  the  life  history  are  the  germination 
of  these  two  spores.  We  have  already  seen  that  the 
result  of  the  germination  is  the  production  of  little 
besides  reproductive  cells,  the  vegetative  parts  of  the 
prothalli  being  very  small  indeed.  In  the  Gymno- 
sperm  the  megaspore  or  embryo  sac  becomes  filled  with 
a  cellular  prothallus — the  endosperm — at  the  upper 
end  of  which  several  archegonia  make  their  appear- 
ance, each  containing  one  ovum.  The  pollen  grain  or 
microspore  puts  out  a  tube  which  bores  its  way  through 
the  substance  of  the  upper  part  of  the  ovule  till  it 
reaches  the  embryo  sac  and  comes  into  contact  with  its 
wall.  As  it  grows  it  produces  two  sperms  in  its  interior ; 
these  are  for  the  most  part  amorphous  pieces  of  proto- 
plasm, though  in  a  few  cases  each  bears  a  band  of  cilia. 
When  the  tube  reaches  the  embryo  sac  the  walls  at  the 
point  of  contact  dissolve  and  the  sperms  pass  through 
and  each  can  fuse  with  an  ovum  in  one  of  the  archegonia. 

In  the  angiospermous  plants  the  process  is  similar, 
but  there  are  characteristic  differences.  The  tubular 


122  BOTANY 

outgrowth  of  the  pollen  grain — the  pollen  tube — pene- 
trates the  style,  reaches  the  cavity  of  the  ovary,  makes 
its  way  to  the  micropyle  of  an  ovule -(Fig.  46).  During 
its  progress  it  develops  two  sperms,  which  are  quite  free 
from  cilia  or  motile  organ  of  any  kind.  The  embryo  sac 
meanwhile  develops  its  prothallus,  which  consists  of  a 
group  of  cells  at  either  end  and  a  large  nucleus  in  the 
centre.  One  of  the  cells  at  the  apical  end  is  the  ovum. 
The  fusion  of  the  walls  of  the  pollen  tube  and  embryo 
sac  takes  place  and  the  two  sperms  enter  the  megaspore. 
One  fuses  with  the  ovum,  the  other  with  the  large 
nucleus  in  the  centre  of  the  sac. 

These  fusions  constitute  what  is  known  as  fertilisation. 


CHAPTER  XVII 

FORMATION    OF   THE    SEED    AND    ITS    MIGRATION— 
THE    FRUIT 

AFTER  a  very  short  interval  further  development  begins. 
The  ovum  in  each  case  clothes  itself  by  a  cell  wall  and 
certain  complicated  divisions  take  place  which  we  cannot 
consider  in  detail  here.  They  result  in  all  cases  in  the 
formation  of  an  embryo,  or  young  sporophyte,  which 
remains  inside  the  embryo  sac.  In  the  Gymnosperm  it 
is  surrounded  from  the  first  by  the  prothallar  tissue; 
in  the  Angiosperm  its  development  is  accompanied  by 
the  formation  from  the  fertilised  nucleus  of  the  embryo 
sac  of  a  similar  mass  of  cells,  also  known  as  the  endo- 
sperm. These  changes  are  accompanied  by  considerable 
growth  of  the  ovule.  Its  integuments  not  only  increase 
in  size  but  become  chemically  altered,  generally  dry  and 
hard.  The  embryo  sac  usually  grows  at  the  expense  of 
the  substance  of  the  ovule  till  it  has  absorbed  the  whole 
of  the  latter  except  the  integuments.  In  some  cases,  as 
the  bean  with  which  we  began  our  study,  the  young 


THE  FRUIT  123 

embryo  absorbs  the  contents  of  the  cells  of  the  endo- 
sperm and  so  fills  the  embryo  sac.  In  others,  as  in  the 
castor  oil  seed,  a  good  deal  of  endosperm  remains ;  in 
yet  others  part  of  the  ovule  may  have  escaped  absorp- 
tion by  the  growing  embryo  sac.  In  all  cases  there  soon 
comes  a  period  when  the  growth  and  development  of  all 
these  parts  stops  and  the  resulting  structure,  now 
become  the  seed,  enters  on  a  more  or  less  prolonged 
period  of  absolute  rest.  This  is  the  period  during  which 
alone  the  migration  of  the  species  is  possible,  as  by 
various  means  the  seed  is  carried  from  the  parent  plant. 

We  called  attention  at  the  outset  to  the  fact  that  the 
life  of  the  individual  plant  has  to  be  spent  absolutely  in 
one  spot,  that  at  which  it  is  rooted  to  the  ground.  How 
disadvantageous  this  is  to  the  plant  and  what  an  endless 
series  of  struggles  against  the  difficulties  it  involves,  has 
been  one  of  the  principal  lines  of  thought  throughout 
our  study.  The  final  difficulty  meets  us  when  we  con- 
sider the  production  of  offspring.  How  can  they 
possibly  flourish  or  even  survive  when  their  parent  is 
hampered  in  this  way  ?  The  solution  of  the  difficulty  is 
found  in  the  provision  which  is  made  for  the  wider  dis- 
persal of  the  reproductive  bodies,  mainly  seeds. 

The  fact  that  most  plants  produce  many  seeds  and 
that  each  seed  after  a  period  of  rest  produces  a  new 
plant  makes  it  imperative  that  adequate  means  of  dis- 
persing seeds  shall  be  found,  or  clearly  the  problem  of 
the  maintenance  of  the  species  would  not  be  solved. 
We  must  turn,  therefore,  to  consider  this  matter  closely 
and  to  study  certain  new  structures  which  are  immedi- 
ately concerned  with  it. 

The  migration  of  seeds  is  brought  about  in  an  almost 
infinite  variety  of  ways,  each  species  having  its  own 
mechanism.  In  most  cases  it  is  associated  with  the 
development  of  a  new  structure,  the  fruit. 

While  the  changes  are  taking  place  by  which  the 


124  BOTANY 

ovule  is  transformed  into  the  seed,  the  stimulus  to 
growth,  which  the  act  of  fertilisation  of  the  ovum  ad- 
ministered to  the  latter,  affects  also  the  parts  in  its 
neighbourhood.  We  have  seen  the  embryo  sac  enlarg- 
ing and  the  integuments  of  the  ovule  developing  into  the 
testa  of  the  seed.  The  ovary,  too,  resumes  development 
and  increases  in  size,  often  enormously,  by  a  large  pro- 
duction of  succulent  cells.  When  its  full  dimensions 
have  been  attained  the  nature  of  the  cells  and  their 
contents  changes.  In  some  cases  they  become  woody, 
hard,  and  dry;  in  others  succulent  and  charged  with 
sugar  and  various  flavouring  matters.  These  latter 
changes  make  up  the  process  known  as  ripening.  The 
enlarged  dry  or  succulent  wall  is  now  known  as  a  peri- 
carp, and  with  the  seeds  which  it  contains  it  constitutes 
the  fruit.  The  latter  is  the  modified  wall  of  the  ovary, 
much  changed  and  developed  by  the  processes  of  growth 
which  have  followed  fertilisation.  The  fruit  is  thus  a 
mechanism  which  is  concerned  chiefly  with  the  problem 
of  the  dispersal  of  the  seeds ;  it  plays  also  an  important 
protective  function  during  the  maturing  of  the  seeds, 
though  this  is  not  its  main  purpose. 

In  some  plants  the  renewed  development  does  not 
stop  at  the  ovary  or  the  carpels.  Other  parts  of  the 
flower  are  involved,  generally  the  axis  or  receptacle,  as 
in  the  apple,  strawberry,  etc.  In  some  cases  the  leaves 
of  the  perianth  also  become  succulent. 

In  yet  other  cases  the  development  affects  simul- 
taneously all  the  flowers  of  a  closely  arranged  inflores- 
cence. All  become  succulent  and  fused  together  to 
form  a  single  fruit,  as  in  the  pine-apple  and  the  fig. 

There  are  thus  many  intricacies  of  development  and 
the  construction  of  the  fruit  is  often  complicated.  Its 
purpose  is  to  distribute  the  seeds  over  a  wide  area. 

The  lines  of  the  development  of  the  fruit  are  in 
almost  all  cases  the  same  at  the  outset.  The  parts 


THE  FRUIT  125 

which  ultimately  form  it  grow,  and  the  new  material 
is  at  first  succulent,  being  made  of  ordinary  thin-walled 
cells.  When  the  full  dimensions  are  attained  this 
succulent  tissue  changes  and  assumes  the  characters  of 
the  ripe  fruit.  Two  main  departures  may  be  noticed — 
in  the  first  the  succulence  becomes  more  pronounced, 
the  cells  more  juicy,  and  their  contents  changed  by  the 
deposition  of  sugar,  the  development  of  particular 
flavours  and  fragrance,  or  of  other  less  attractive 
chemical  substances.  We  get  thus  a  class  of  fruits 
which  appeal  to  the  animal  world  and  whose  fate  is 
probably  to  be  eaten.  The  seeds  which  are  developed 
inside  them  are  usually  furnished  with  a  hard  testa  or 
skin,  so  that  they  may  escape  injury  in  passing  through 
the  animal's  body.  Succulent  and  hard  parts  thus  go 
together,  though  their  soft  and  their  resistant  parts 
have  not  in  all  cases  the  same  origin. 

In  the  second  departure  from  the  original  softness  we 
find  a  tendency  to  hardness  and  dry  ness  throughout. 
Sometimes  the  fruit  becomes  woody — more  frequently 
dry  and  papery,  or  resembling  cork  in  its  general  pro- 
perties. This  type  of  fruit  is  associated  with  other 
means  of  dispersal :  often  endowed  with  a  kind  of  explo- 
sive mechanism,  so  that  rupture  of  its  walls  is  followed 
by  a  jerking  of  the  seed  for  some  distance;  often 
furnished  with  some  means  of  transport,  such  as  hooks, 
that  may  attach  themselves  to  passing  animals,  or 
floats  of  various  kinds  that  may  buoy  up  the  fruit  in  the 
air  and  enable  it  to  take  advantage  of  currents  of  wind. 

In  some  cases  the  seeds  themselves  are  furnished 
with  one  or  other  of  these  mechanisms.  It  is,  indeed, 
often  difficult  to  distinguish  between  small  fruits  and 
seeds  when  the  latter  are  thus  endowed.  In  such  cases 
they  always  escape  from  the  fruit  before  dispersal. 

Various  methods  of  classifying  fruits  have  been 
adopted,  and  a  somewhat  ponderous  nomenclature  has 


126  BOTANY 

arisen,  which,  however,  is  comparatively  unimportant. 
The  important  consideration  is  the  need  of  the  plant; 
the  various  ways  in  which  it  is  supplied  may  advan- 
tageously occupy  our  thoughts  rather  than  the  duty  of 
finding  a  special  name  for  each  variety.  It  has,  how- 
ever, become  the  custom  to  speak  of  a  fruit  -which  has 
been  developed  from  the  carpel  or  carpels  only  of  a 
flower  as  a  true  fruit.  One  into  whose  composition  some 
other  part  of  the  flower  enters,  usually  some  part  of  the 
floral  axis,  is  known  as  a  spurious  fruit,  or  pseudocarp. 
The  distinction  is  in  many  cases  very  difficult  to  make 
and  from  our  point  of  view  is  quite  unnecessary.  Such 
fruits  as  the  pine-apple  and  fig,  which  are  the  product  of 
whole  inflorescences  and  not  of  single  flowers,  may  be 
distinguished  as  aggregated  fruits. 

Among  the  succulent  fruits  the  most  prominent  is 
the  berry,  which  exists  in  several  varieties.  It  is  seen 
in  its  simplest  form  in  the  grape,  while  varieties  of  it  are 
illustrated  by  the  gooseberry  and  the  orange.  The 
hard  parts  of  this  mechanism  are  the  walls  of  the  seeds 
the  berries  contain.  Another  succulent  fruit  is  the 
drupe,  in  which  the  middle  layer  of  the  fruit  wall  be- 
comes pulpy  while  an  inner  layer  becomes  hard  and 
constitutes  the  stone.  A  collection  of  very  small 
drupes  upon  a  dry  receptacle  is  met  with  in  the  different 
kinds  of  raspberry  and  blackberry. 

Succulent  fruits  in  which  the  growing  axis  of  the 
flower  is  concerned  are  met  with  in  two  conspicuous 
forms.  The  strawberry  has  a  very  succulent  convex 
axis  on  which  the  fruits  appear  as  small  hard  bodies, 
containing,  however,  the  seeds  inside  their  hard  coats; 
in  the  apple  and  its  allies  the  succulent  axis  has  become 
concave  and  has  grown  up  round  the  carpels  and  enclosed 
them.  This  fruit  is  called  a  pome.  The  carpels  them- 
selves are  cartilaginous  in  texture,  or  in  some  forms 
bony — as  in  the  hawthorn. 


THE  FRUIT  127 

Dry  fruits  show  greater  variety.  In  some  cases  they 
consist  of  one  carpel  only,  many  such  fruits  arising  from 
a  single  flower,  as  in  the  buttercup.  This  single  carpel 
may  remain  permanently  closed,  the  seed  being  set  free 
only  by  its  decay,  or  it  may  open  along  its  front  or  both 
front  and  back  margins.  In  the  latter  case  many  seeds 
are  generally  found  inside.  In  other  cases  the  carpels 
while  in  the  flower  are  joined  together  in  their  develop- 
ment. When  such  a  pistil  is  cut  across  it  very  generally 
shows  as  many  cavities  as  there  are  constituent  carpels. 
Sometimes  the  walls  of  the  latter  do  not  meet  in  the 
centre,  so  that  there  is  but  one  cavity.  The  seeds  are 
attached  with  hardly  any  exceptions  to  an  outgrowth 
of  the  edge  of  the  carpellary  leaf  which  constitutes  a 
placenta.  There  is  a  good  deal  of  variety  in  this  form 
of  fruit,  as  it  varies  with  the  number  of  carpels  and 
the  ways  in  which  they  are  united.  Such  fruits  are 
commonly  called  capsules.  A  singular  variety  of  the 
capsule  is  seen  in  the  fruit  of  the  wallflower  and  its 
allies.  Two  carpels  are  joined  together  by  their  edges 
and  at  the  lines  of  union  where  a  placenta  can  be  seen 
a  membrane  is  developed  quite  across  the  cavity. 

Besides  the  dry  fruits  which  open  and  let  the  seeds 
out  and  those  which  retain  the  latter  permanently, 
another  form  is  met  with,  consisting  of  several  united 
carpels,  which  separate  from  each  other  at  maturity, 
but  each  carpel  continues  to  hold  its  seed.  These 
fruits  are  called  schizocarps. 

A  modification  of  the  polycarpillary  fruit  is  seen  in  the 
nut.  It  has  two  or  three  constituent  carpels  and  the 
young  fruit  shows  as  many  cavities  as  carpels.  In 
development,  however,  some  or  all  of  the  partition  walls 
become  dried  up  and  disappear,  so  that  there  is  only  one 
cavity  in  the  adult  fruit  and  this  seldom  contains  more 
than  one  seed.  It  is  associated  with  a  very  hard 
woody  wall.  All  these  modifications  of  structure  show 


128  BOTANY 

particular  adaptations  to  the  mode  of  dispersal  which  the 
plant  has  adopted  and  should  be  studied  mainly  from 
this  point  of  view. 

Small  fruits  and  seeds  are  blown  about  by  the  wind 
or  carried  by  birds.  Some  are  furnished  with  buoyant 
accessories,  which  enable  them  to  remain  in  the  air 
for  considerable  periods.  In  some  cases  the  fruit  splits 
open  with  explosive  violence  and  the  seeds  are  jerked 
to  some  distance.  Often  small  fruits  or  seeds  are 
carried  long  distances  embedded  in  mud,  into  which 
they  have  fallen  and  which  has  subsequently  become 
attached  to  the  feet  of  birds  or  other  animals.  Larger 
fruits  become  attached  by  means  of  hook-like  append- 
ages to  the  coats  of  similar  wanderers.  Many  fruits  are 
capable  of  floating  long  distances  in  water — are  indeed 
often  furnished  with  special  mechanisms  to  enable 
them  to  float.  Indeed,  the  means  of  dispersal  are  so 
numerous  and  in  many  cases  so  intricate  that  it  is  im- 
possible here  to  do  more  than  indicate  the  merest  out- 
lines of  the  subject.  The  mechanisms  are  easy  to  study 
and  every  plant  one  meets  affords  an  example  which 
will  well  repay  investigation. 


THE    TEMPLE    PRESS,    PRINTERS,    LETCHWORTH 


MM    I  1,  /V 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
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OVERDUE. 


moil 

AUG  29  1938 

°CT  2  7  J941 

NOV  2  6  1941 

' 

APR  7    1966 

•  ">'    Ft      *            "VMV 

24«T'«>*H 

U.C.  BERKELEY  LIBRARIES 


€03^711710 


911004 


THE  UNIVERSITY  OF  CALIFORNIA  LIBRARY 


